<controller type='pci' index='0' model='pci-root'/>
is auto-added to pc* machine types.
Without this controller PCI bus 0 is not available and
no PCI addresses are assigned by default.
Since older libvirt supported PCI bus 0 even without
this controller, it is removed from the XML when migrating.
Instead of making a choice between the underscore and camelCase, this
simply changes "num_queues" into "queues", which is also consistent
with Michal's multiple queue support for interface.
This introduce a new attribute "num_queues" (same with the good name
QEMU uses) for virtio-scsi controller. An example of the XML:
<controller type='scsi' index='0' model='virtio-scsi' num_queues='8'/>
The corresponding QEMU command line:
-device virtio-scsi-pci,id=scsi0,num_queues=8,bus=pci.0,addr=0x3 \
