virtio-blk: dataplane multiqueue support

virtio-blk: dataplane multiqueue support

Monitor ioeventfds for all virtqueues in the device's AioContext.  This
is not true multiqueue because requests from all virtqueues are
processed in a single IOThread.  In the future it will be possible to
use multiple IOThreads when the QEMU block layer supports multiqueue.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466511196-12612-7-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>