virtio-blk: multiqueue batch notify

virtio-blk: multiqueue batch notify

The batch notification BH needs to know which virtqueues to notify when
multiqueue is enabled.  Use a bitmap to track the virtqueues with
pending notifications.

At this point there is only one virtqueue so hard-code virtqueue index
0.  A later patch will switch to real virtqueue indices.

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]>