virtio-blk: live migrate s->rq with multiqueue

virtio-blk: live migrate s->rq with multiqueue

Add a field for the virtqueue index when migrating the s->rq request
list.  The new field is only needed when num_queues > 1.  Existing QEMUs
are unaffected by this change and therefore virtio-blk migration stays
compatible.

Suggested-by: Paolo Bonzini <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: 1466511196[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>