This adds a test case that the BlockDriver callbacks for drain are
called in bdrv_drained_all_begin/end(), and that both of them are called
exactly once.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>