The '-usb' option doesn't have any effect for aarch64 mach-virt
guests, so the fact that it's currently enabled by default is not
really causing any issue.
However, that might change in the future (although unlikely), and
having it as part of the QEMU command line can cause confusion to
someone looking through the process list.
Avoid it completely, like it's already happening for q35.
There has been some progress lately in enabling virtio-pci on
aarch64 guests; however, guest OS support is still spotty at best,
so most guests are going to be using virtio-mmio instead.
Currently, mach-virt guests are closely modeled after q35 guests,
and that includes always adding a dmi-to-pci-bridge that's just
impossible to get rid of. While that's acceptable (if suboptimal)
for q35, where you will always need some kind of PCI device anyway,
mach-virt guests should be allowed to avoid it.
Until now, a Q35 domain (or arm/virt, or any other domain that has a
pcie-root bus) would always have a pci-bridge added, so that there
would be a hotpluggable standard PCI slot available to plug in any PCI
devices that might be added. This patch removes the explicit add,
instead relying on the pci-bridge being auto-added during PCI address
assignment (it will add a pci-bridge if there are no free slots).
This doesn't eliminate the dmi-to-pci-bridge controller that is
explicitly added whether or not a standard PCI slot is required (and
that is almost never used as anything other than a converter between
pcie.0's PCIe slots and standard PCI). That will be done separately.
When support for dmi-to-pci-bridge was added, it was assumed that,
just as with the pci-root bus, slot 0 was reserved. This is not the
case - it can be used to connect a device just like any other slot, so
remove the restriction and update the test cases that auto-assign an
address on a dmi-to-pci-bridge.
Some places already check for "virt-" prefix as well as plain "virt".
virQEMUCapsHasPCIMultiBus did not, resulting in multiple PCI devices
having assigned the same unnumbered "pci" alias.
Add a test for the "virt-2.6" machine type which also omits the
<model type='virtio'/> in <interface>, to check if
qemuDomainDefaultNetModel works too.
https://bugzilla.redhat.com/show_bug.cgi?id=1325085
