qemu 2.7.0 introduces multiqueue virtio-blk(commit 2f27059).
This patch introduces a new attribute "queues". An example of
the XML:

<disk type='file' device='disk'>
  <driver name='qemu' type='qcow2' queues='4'/>

The corresponding QEMU command line:

-device virtio-blk-pci,scsi=off,num-queues=4,id=virtio-disk0
Signed-off-by: NLin Ma <lma@suse.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>