This introduces two new attributes "cmd_per_lun" and "max_sectors" same
with the names QEMU uses for virtio-scsi. An example of the XML:

<controller type='scsi' index='0' model='virtio-scsi' cmd_per_lun='50'
max_sectors='512'/>

The corresponding QEMU command line:

-device virtio-scsi-pci,id=scsi0,cmd_per_lun=50,max_sectors=512,
bus=pci.0,addr=0x3
Signed-off-by: NMike Perez <thingee@gmail.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>