dataplane: enable virtio-blk x-data-plane=on live migration

dataplane: enable virtio-blk x-data-plane=on live migration

Although the dataplane thread does not cooperate with dirty memory
logging yet it's fairly easy to temporarily disable dataplane during
live migration.  This way virtio-blk can live migrate when
x-data-plane=on.

The dataplane thread will restart after migration is cancelled or if the
guest resuming virtio-blk operation after migration completes.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>