virtio: update queue size on guest write

virtio: update queue size on guest write

Some guests read back queue size after writing it.
Update the size immediatly upon write otherwise
they get confused.

In particular this is the case for seabios.

Reported-by: Roman Kagan <rkagan@virtuozzo.com>
Suggested-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit d0c5f643383b9e84316f148affff368ac33d75b9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>