Commit f7afeddc added code to report to systemd an array of interface
indexes for all tap devices used by a guest. Unfortunately it not only
didn't add code to report the ifindexes for macvtap interfaces
(interface type='direct') or the tap devices used by type='ethernet',
it ended up sending "-1" as the ifindex for each macvtap or hostdev
interface. This resulted in a failure to start any domain that had a
macvtap or hostdev interface (or actually any type other than
"network" or "bridge").
This patch does the following with the nicindexes array:
1) Modify qemuBuildInterfaceCommandLine() to only fill in the
nicindexes array if given a non-NULL pointer to an array (and modifies
the test jig calls to the function to send NULL). This is because
there are tests in the test suite that have type='ethernet' and still
have an ifname specified, but that device of course doesn't actually
exist on the test system, so attempts to call virNetDevGetIndex() will
fail.
2) Even then, only add an entry to the nicindexes array for
appropriate types, and to do so for all appropriate types ("network",
"bridge", and "direct"), but only if the ifname is known (since that
is required to call virNetDevGetIndex().
Export the required helpers and add backend code to hotplug RNG devices.
Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Rename qemuBuildRNGDeviceArgs to qemuBuildRNGDevStr and change the
return type so that it can be reused in the device hotplug code later.
Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
This function is used to assign an alias for a RNG device. It will be
later reused when hotplugging RNGs.
Signed-off-by: Luyao Huang <lhuang@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Unlike -device, qemu uses a JSON object to add backend "objects" via the
monitor rather than the string that would be passed on the commandline.
To be able to reuse code parts that configure backends for various
devices, this patch adds a helper that will allow generating the command
line representations from the JSON property object.
In order to be able to test for fully reserved PCI buses, assignment of
PCI slots for integrated devices needs to be moved to a separate function.
This also might be a good preparation if we decide to add support for
other chipsets as well.
The default value should be 16 MiB instead of 8 MiB. Only really old
version of upstream QEMU used the 8 MiB as default for vga framebuffer.
Without this change if you update your libvirt where we introduced the
"vgamem" attribute for QXL video device the value will be set to 8 MiB,
but previously your guest had 16 MiB because we didn't pass any value to
QEMU command line which means QEMU used its own 16 MiB as default.
This will affect all users with guest's display resolution higher than
1920x1080.
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Particularly in qemuBuildNumaArgStr(), there was a need for the advice
due to memory backing, which needs to know the nodeset it will be pinned
to. With newer qemu this caused the following error when starting
domain:
error: internal error: Advice from numad is needed in case of
automatic numa placement
even when starting perfectly valid domain, e.g.:
...
<vcpu placement='auto'>4</vcpu>
<numatune>
<memory mode='strict' placement='auto'/>
</numatune>
<cpu>
<numa>
<cell id='0' cpus='0' memory='524288'/>
<cell id='1' cpus='1' memory='524288'/>
</numa>
</cpu>
...
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1138545
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
We are not detecting the presence of FIPS from QEMU, but from procfs and
that means it's not QEMU capability. It was decided that we will pass
this flag to QEMU even if it's not supported by old QEMU binaries.
This patch also reverts changes done by commit a21cfb0f to
qemucapabilitestest and implements a new test case in qemuxml2argvtest.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1135431
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
https://bugzilla.redhat.com/show_bug.cgi?id=1078126
Using 'virsh attach-device --config' (or --persistent) to attach a
file backed lun device will succeed; however, subsequent domain restarts
will result in failure because the configuration of a file backed lun
is not supported.
Although allowing 'illegal configurations' is something that can be
allowed, it may not be practical in this case. Generally, when attaching
a device to a domain means the domain must be running. A way around
this is using the --config (or --persistent) option. When an attach
is done to a running domain, a temporary configuration is modified
first followed by the live update. The live update will make a number
of disk validity checks when building the qemu command to attach the
disk. If any fail, then change is rejected.
Rather than allow a potentially illegal combination, adjust the code
in the configuration path to make the same checks as the running path
will make with respect to disk validity checks. This way we avoid
having the potential for some subsequent start/reboot to fail because
an illegal combination was allowed.
NB: The live path still checks the configuration since it is possible
to just do --live guest modification...
During review of the iSCSI hostdev series, eblake noted that the
prototypes shouldn't have the extranenous space between the "*" and
the function name:
http://www.redhat.com/archives/libvir-list/2014-July/msg01227.html
Since it was more invasive than 1 or 2 lines - I said I'd send a
patch covering this once committed.
Signed-off-by: John Ferlan <jferlan@redhat.com>
Move sharable PCI handling functions to domain_addr.[ch], and
change theirs prefix from 'qemu' to 'vir':
- virDomainPCIAddressAsString;
- virDomainPCIAddressBusSetModel;
- virDomainPCIAddressEnsureAddr;
- virDomainPCIAddressFlagsCompatible;
- virDomainPCIAddressGetNextSlot;
- virDomainPCIAddressReleaseSlot;
- virDomainPCIAddressReserveAddr;
- virDomainPCIAddressReserveNextSlot;
- virDomainPCIAddressReserveSlot;
- virDomainPCIAddressSetFree;
- virDomainPCIAddressSetGrow;
- virDomainPCIAddressSlotInUse;
- virDomainPCIAddressValidate;
The only change here is function names, the implementation itself
stays untouched.
Extract common allocation code from DomainPCIAddressSetCreate
into virDomainPCIAddressSetAlloc.
Introduce new files (domain_addr.[ch]) to provide
an API for domain device handling that could be
shared across the drivers.
A list of data types were extracted and moved there:
qemuDomainPCIAddressBus -> virDomainPCIAddressBus
qemuDomainPCIAddressBusPtr -> virDomainPCIAddressBusPtr
_qemuDomainPCIAddressSet -> virDomainPCIAddressSet
qemuDomainPCIAddressSetPtr -> virDomainPCIAddressSetPtr
qemuDomainPCIConnectFlags -> virDomainPCIConnectFlags
Also, move the related definitions and macros.
This uses the new QEMU_CAPS_HOST_PCI_MULTIDOMAIN capability when
present, for -devivce pci-assign, -device vfio-pci, and -pcidevice.
While creating tests for this new functionality, I noticed that the
xmls for two existing tests had erroneously specified an
until-now-ignored domain="0x0002", so I corrected those two tests, and
also added two failure tests to be sure that we alert users who
attempt to use a non-zero domain with a qemu that doesn't support it.
In "src/util/" there are many enumeration (enum) declarations.
Sometimes, it's better using a typedef for variable types,
function types and other usages. Other enumeration will be
changed to typedef's in the future.
Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Refactor the function to avoid multiple wrappers splitting identical
fields from the now common metadata struct.
The refactor is done by folding in the wrapper used for disk sources
which allows us to lookup secrets via the secret driver. This may allow
using stored secrets for snapshot disk images too in the future.
Since it is an abbreviation, USB should always be fully
capitalized or full lower case, never Usb.
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Add support for gluster backed images as sources for snapshots in the
qemu driver. This will also simplify adding further network backed
volumes as sources for snapshot in case qemu will support them.
To support passing the path of the test data to the utils, one
more argument is added to virSCSIDeviceGetSgName,
virSCSIDeviceGetDevName, and virSCSIDeviceNew, and the related
code is changed accordingly.
Later tests for the scsi utils will be based on this patch.
Signed-off-by: Osier Yang <jyang@redhat.com>
https://bugzilla.redhat.com/show_bug.cgi?id=1019053
When we migrate vms concurrently, there's a chance that libvirtd on
destination assigns the same port for different migrations, which will
lead to migration failure during prepare phase on destination. So we use
virPortAllocator here to solve the problem.
Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Part of the resolution to:
https://bugzilla.redhat.com/show_bug.cgi?id=1003983
Although most devices available in qemu area defined as PCI devices,
and strictly speaking should only be attached via a PCI slot, in
practice qemu allows them to be attached to a PCIe slot and sometimes
this makes sense.
For example, The UHCI and EHCI USB controllers are usually attached
directly to the PCIe "root complex" (i.e. PCIe slots) on real
hardware, so that should be possible for a Q35-based qemu virtual
machine as well.
We still want to prefer a standard PCI slot when auto-assigning
addresses, though, and in general to disallow attaching PCI devices
via PCIe slots.
This patch makes that possible by adding a new
QEMU_PCI_CONNECT_TYPE_EITHER_IF_CONFIG flag. Three things are done
with this flag:
1) It is set for the "pcie-root" controller
2) qemuCollectPCIAddress() now has a set of nested switches that set
this "EITHER" flag for devices that we want to allow connecting to
pcie-root when specifically requested in the config.
3) qemuDomainPCIAddressFlagsCompatible() adds this new flag to the
"flagsMatchMask" if the address being checked came from config rather
than being newly auto-allocated by libvirt (this knowledge is
conveniently already available in the "fromConfig" arg).
Now any device having the EITHER flag set can be connected to
pcie-root if explicitly requested, but auto-allocated addresses for
those devices will still be standard PCI slots instead.
This patch only loosens the restrictions on devices that have been
specifically requested, but the setup is such that it should be fairly
easy to add new devices.
Most callers of qemuParseKeywords were assigning its return
value to a 'size_t' variable. Then then also checked '< 0'
for error condition, but this will never be true with the
unsigned size_t variable. Rather than using 'ssize_t', change
qemuParseKeywords so that the element count is returned via
an output parameter, leaving the return value solely as an
error indicator.
This avoids a crash accessing beyond the end of an error
upon OOM.
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
No need to open code now that we have a nice function.
Interestingly, our virStringFreeList function is typed correctly
(a malloc'd list of malloc'd strings is NOT const, whether at the
point where it is created, or at the point where it is cleand up),
so using it with a 'const char **' argument would require a cast
to keep the compiler. I chose instead to remove const from code
even where we don't modify the argument, just to avoid the need
to cast.
* src/qemu/qemu_command.h (qemuParseCommandLine): Drop declaration.
* src/qemu/qemu_command.c (qemuParseProcFileStrings)
(qemuStringToArgvEnv): Don't force malloc'd result to be const.
(qemuParseCommandLinePid, qemuParseCommandLineString): Simplify
cleanup.
(qemuParseCommandLine, qemuFindEnv): Drop const-correctness to
avoid the need to cast in callers.
Signed-off-by: Eric Blake <eblake@redhat.com>
If user requested multiqueue networking, beside multiple /dev/tap and
/dev/vhost-net openings, we forgot to pass mq=on onto the -device
virtio-net-pci command line. This is advised at:
http://www.linux-kvm.org/page/Multiqueue#Enable_MQ_feature
We had been setting the device alias in the devinceinfo for pci
controllers to "pci%u", but then hardcoding "pci.%u" when creating the
device address for other devices using that pci bus. This all worked
just fine until we encountered the built-in "pcie.0" bus (the PCIe
root complex) in Q35 machines.
In order to create the correct commandline for this one case, this
patch:
1) sets the alias for PCI controllers correctly, to "pci.%u" (or
"pcie.%u" for the pcie-root controller)
2) eliminates the hardcoded "pci.%u" for pci controllers when
generatuing device address strings, and instead uses the controller's
alias.
3) plumbs a pointer to the virDomainDef all the way down to
qemuBuildDeviceAddressStr. This was necessary in order to make the
aliase of the controller *used by a device* available (previously
qemuBuildDeviceAddressStr only had the deviceinfo of the device
itself, *not* of the controller it was connecting to). This made for a
larger than desired diff, but at least in the future we won't have to
do it again, since all the information we could possibly ever need for
future enhancements is in the virDomainDef. (right?)
This should be done for *all* controllers, but for now we just do it
in the case of PCI controllers, to reduce the likelyhood of
regression.
This controller is implicit on q35 machinetypes. It provides 31 PCIe
(*not* PCI) slots as controller 0.
Currently there are no devices that can connect to pcie-root, and no
implicit pci controller on a q35 machine, so q35 is still
unusable. For a usable q35 system, we need to add a
"dmi-to-pci-bridge" pci controller, which can connect to pcie-root,
and provides standard pci slots that can be used to connect other
devices.
* The functions qemuDomainPCIAddressReserveAddr and
qemuDomainPCIAddressReserveSlot were very similar (and should have
been more similar) and were about to get more code added to them which
would create even more duplicated code, so this patch gives
qemuDomainPCIAddressReserveAddr a "reserveEntireSlot" arg, then
replaces the body of qemuDomainPCIAddressReserveSlot with a call to
qemuDomainPCIAddressReserveAddr.
You will notice that addrs->lastaddr was previously set in
qemuDomainPCIAddressReserveAddr (but *not* set in
qemuDomainPCIAddressReserveSlot). For consistency and cleanliness of
code, that bit was removed and put into the one caller of
qemuDomainPCIAddressReserveAddr (there is a similar place where the
caller of qemuDomainPCIAddressReserveSlot sets lastaddr). This does
guarantee identical functionality to pre-patch code, but in practice
isn't really critical, because lastaddr is just keeping track of where
to start when looking for a free slot - if it isn't updated, we will
just start looking on a slot that's already occupied, then skip up to
one that isn't.
* qemuCollectPCIAddress was essentially doing the same thing as
qemuDomainPCIAddressReserveAddr, but with some extra special case
checking at the beginning. The duplicate code has been replaced with
a call to qemuDomainPCIAddressReserveAddr. This required adding a
"fromConfig" boolean, which is only used to change the log error
code from VIR_ERR_INTERNAL_ERROR (when the address was
auto-generated by libvirt) to VIR_ERR_XML_ERROR (when the address is
coming from the config); without this differentiation, it would be
difficult to tell if an error was caused by something wrong in
libvirt's auto-allocate code or just bad config.
* the bit of code in qemuDomainPCIAddressValidate that checks the
connect type flags is going to be used in a couple more places where
we don't need to also check the slot limits (because we're generating
the slot number ourselves), so that has been pulled out into a
separate qemuDomainPCIAddressFlagsCompatible function.
* qemuDomainPCIAddressSetNextAddr
The name of this function was confusing because 1) other functions in
the file that end in "Addr" are only operating on a single function of
one PCI slot, not the entire slot, while functions that do something
with the entire slot end in "Slot", and 2) it didn't contain a verb
describing what it is doing (the "Set" refers to the set that contains
all PCI buses in the system, used to keep track of which slots in
which buses are already reserved for use).
It is now renamed to qemuDomainPCIAddressReserveNextSlot, which more
clearly describes what it is doing. Arguably, it could have been
changed to qemuDomainPCIAddressSetReserveNextSlot, but 1) the word
"set" is confusing in this context because it could be intended as a
verb or as a noun, and 2) most other functions that operate on a
single slot or address within this set are also named
qemuDomainPCIAddress... rather than qemuDomainPCIAddressSet... Only
the Create, Free, and Grow functions for an address set (which modify the
entire set, not just one element) use "Set" in their name.
* qemuPCIAddressAsString, qemuPCIAddressValidate
All the other functions in this set are named
qemuDomainPCIAddressxxxxx, so I renamed these to be consistent.
Since PCI bridges, PCIe bridges, PCIe switches, and PCIe root ports
all share the same namespace, they are all defined as controllers of
type='pci' in libvirt (but with a differing model attribute). Each of
these controllers has a certain connection type upstream, allows
certain connection types downstream, and each can either allow a
single downstream connection at slot 0, or connections from slot 1 -
31.
Right now, we only support the pci-root and pci-bridge devices, both
of which only allow PCI devices to connect, and both which have usable
slots 1 - 31. In preparation for adding other types of controllers
that have different capabilities, this patch 1) adds info to the
qemuDomainPCIAddressBus object to indicate the capabilities, 2) sets
those capabilities appropriately for pci-root and pci-bridge devices,
and 3) validates that the controller being connected to is the proper
type when allocating slots or validating that a user-selected slot is
appropriate for a device..
Having this infrastructure in place will make it much easier to add
support for the other PCI controller types.
While it would be possible to do all the necessary checking by just
storing the controller model in the qemyuDomainPCIAddressBus, it
greatly simplifies all the validation code to also keep a "flags",
"minSlot" and "maxSlot" for each - that way we can just check those
attributes rather than requiring a nearly identical switch statement
everywhere we need to validate compatibility.
You may notice many places where the flags are seemingly hard-coded to
QEMU_PCI_CONNECT_HOTPLUGGABLE | QEMU_PCI_CONNECT_TYPE_PCI
This is currently the correct value for all PCI devices, and in the
future will be the default, with small bits of code added to change to
the flags for the few devices which are the exceptions to this rule.
Finally, there are a few places with "FIXME" comments. Note that these
aren't indicating places that are broken according to the currently
supported devices, they are places that will need fixing when support
for new PCI controller models is added.
To assure that there was no regression in the auto-allocation of PCI
addresses or auto-creation of integrated pci-root, ide, and usb
controllers, a new test case (pci-bridge-many-disks) has been added to
both the qemuxml2argv and qemuxml2xml tests. This new test defines a
domain with several dozen virtio disks but no pci-root or
pci-bridges. The .args file of the new test case was created using
libvirt sources from before this patch, and the test still passes
after this patch has been applied.
The function being introduced is responsible for creating command
line argument for '-device' for given character device. Based on
the chardev type, it calls appropriate qemuBuild.*ChrDeviceStr(),
e.g. qemuBuildSerialChrDeviceStr() for serial chardev and so on.
The chardev alias assignment is going to be needed in a separate
places, so it should be moved into a separate function rather
than copying code randomly around.
In order to learn libvirt multiqueue several things must be done:
1) The '/dev/net/tun' device needs to be opened multiple times with
IFF_MULTI_QUEUE flag passed to ioctl(fd, TUNSETIFF, &ifr);
2) Similarly, '/dev/vhost-net' must be opened as many times as in 1)
in order to keep 1:1 ratio recommended by qemu and kernel folks.
3) The command line construction code needs to switch from 'fd=X' to
'fds=X:Y:...:Z' and from 'vhostfd=X' to 'vhostfds=X:Y:...:Z'.
4) The monitor handling code needs to learn to pass multiple FDs.
Since 0d70656afd, it starts to access the sysfs files to build
the qemu command line (by virSCSIDeviceGetSgName, which is to find
out the scsi generic device name by adpater🚌target:unit), there
is no way to work around, qemu wants to see the scsi generic device
like "/dev/sg6" anyway.
And there might be other places which need to access sysfs files
when building qemu command line in future.
Instead of increasing the arguments of qemuBuildCommandLine, this
introduces a new callback for qemuBuildCommandLine, and thus tests
can register their own callbacks for sysfs test input files accessing.
* src/qemu/qemu_command.h: (New callback struct
qemuBuildCommandLineCallbacks;
extern buildCommandLineCallbacks)
* src/qemu/qemu_command.c: (wire up the callback struct)
* src/qemu/qemu_driver.c: (Use the new syntax of qemuBuildCommandLine)
* src/qemu/qemu_hotplug.c: Likewise
* src/qemu/qemu_process.c: Likewise
* tests/testutilsqemu.[ch]: (Helper testSCSIDeviceGetSgName;
callback struct testCallbacks;)
* tests/qemuxml2argvtest.c: (Use testCallbacks)
* src/tests/qemuxmlnstest.c: (Like above)
Adding a VNC WebSocket support for QEMU driver. This functionality is
in upstream qemu from commit described as v1.3.0-982-g7536ee4, so the
capability is being recognized based on QEMU version for now.
Except the scsi host device's controller is "lsilogic", mapping
between the libvirt attributes and scsi-generic properties is:
libvirt qemu
-----------------------------------------
controller bus ($libvirt_controller.0)
bus channel
target scsi-id
unit lun
For scsi host device with "lsilogic" controller, the mapping is:
('target (libvirt)' must be 0, as it's not used; 'unit (libvirt)
must <= 7).
libvirt qemu
----------------------------------------------------------
controller && bus bus ($libvirt_controller.$libvirt_bus)
unit scsi-id
It's not good to hardcode/hard-check limits of these attributes,
and even worse, these limits are not documented, one has to find
out by either testing or reading the qemu code, I'm looking forward
to qemu expose limits like these one day). For example, exposing
"max_target", "max_lun" for megasas:
static const struct SCSIBusInfo megasas_scsi_info = {
.tcq = true,
.max_target = MFI_MAX_LD,
.max_lun = 255,
.transfer_data = megasas_xfer_complete,
.get_sg_list = megasas_get_sg_list,
.complete = megasas_command_complete,
.cancel = megasas_command_cancel,
};
Example of the qemu command line (lsilogic controller):
-drive file=/dev/sg2,if=none,id=drive-hostdev-scsi_host7-0-0-0 \
-device scsi-generic,bus=scsi0.0,scsi-id=8,\
drive=drive-hostdev-scsi_host7-0-0-0,id=hostdev-scsi_host7-0-0-0
Example of the qemu command line (virtio-scsi controller):
-drive file=/dev/sg2,if=none,id=drive-hostdev-scsi_host7-0-0-0 \
-device scsi-generic,bus=scsi0.0,channel=0,scsi-id=128,lun=128,\
drive=drive-hostdev-scsi_host7-0-0-0,id=hostdev-scsi_host7-0-0-0
Signed-off-by: Han Cheng <hanc.fnst@cn.fujitsu.com>
Signed-off-by: Osier Yang <jyang@redhat.com>
VFIO device assignment requires a cgroup ACL to be setup for access to
the /dev/vfio/nn "group" device for any devices that will be assigned
to a guest. In the case of a host device that is allocated from a
pool, it was being allocated during qemuBuildCommandLine(), which is
called by qemuProcessStart() *after* the all-encompassing
qemuSetupCgroup() was called, meaning that the standard Cgroup ACL
setup wasn't creating ACLs for these devices allocated from pools.
One possible solution was to manually add a single ACL down inside
qemuBuildCommandLine() when networkAllocateActualDevice() is called,
but that has two problems: 1) the function that adds the cgroup ACL
requires a virDomainObjPtr, which isn't available in
qemuBuildCommandLine(), and 2) we really shouldn't be doing network
device setup inside qemuBuildCommandLine() anyway.
Instead, I've created a new function called
qemuNetworkPrepareDevices() which is called just before
qemuPrepareHostDevices() during qemuProcessStart() (explanation of
ordering in the comments), i.e. well before the call to
qemuSetupCgroup(). To minimize code churn in a patch that will be
backported to 1.0.5-maint, qemuNetworkPrepareDevices only does
networkAllocateActualDevice() and the bare amount of setup required
for type='hostdev network devices, but it eventually should do *all*
device setup for guest network devices.
Note that some of the code that was previously needed in
qemuBuildCommandLine() is no longer required when
networkAllocateActualDevice() is called earlier:
* qemuAssignDeviceHostdevAlias() is already done further down in
qemuProcessStart().
* qemuPrepareHostdevPCIDevices() is called by
qemuPrepareHostDevices() which is called after
qemuNetworkPrepareDevices() in qemuProcessStart().
As hinted above, this new function should be moved into a separate
qemu_network.c (or similarly named) file along with
qemuPhysIfaceConnect(), qemuNetworkIfaceConnect(), and
qemuOpenVhostNet(), and expanded to call those functions as well, then
the nnets loop in qemuBuildCommandLine() should be reduced to only
build the commandline string (which itself can be in a separate
qemuInterfaceBuilldCommandLine() function as suggested by
Michal). However, this will require storing away an array of tapfd and
vhostfd that are needed for the commandline, so I would rather do that
in a separate patch and leave this patch at the minimum to fix the
bug.
<source type='bridge'> uses a helper application to do the necessary
TUN/TAP setup to use an existing network bridge, thus letting
unprivileged users use TUN/TAP interfaces.
However, libvirt should be preventing QEMU from running any setuid
programs at all, which would include this helper program. From
a security POV, any setuid helper needs to be run by libvirtd itself,
not QEMU.
This is what this patch does. libvirt now invokes the setuid helper,
gets the TAP fd and then passes it to QEMU in the normal manner.
The path to the helper is specified in qemu.conf.
As a small advantage, this adds a <target dev='tap0'/> element to the
XML of an active domain using <interface type='bridge'>.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
