virtio-blk: dataplane: notify guest as a batch

Now requests are submitted as a batch, so it is natural
to notify guest as a batch too.

This may suppress interrupt notification to VM a lot:

        - in my test, decreased by ~13K/sec

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>