This patch implements .bdrv_io_plug, .bdrv_io_unplug and
.bdrv_flush_io_queue callbacks for linux-aio Block Drivers,
so that submitting I/O as a batch can be supported on linux-aio.

[Unprocessed requests are completed with -EIO instead of a bogus ret
value.
--Stefan]
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>