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 \
