This patch adds test cases for the scenario where blk_aio_flush() is
called on a BlockBackend with no root.  Calling drain afterwards should
complete the requests with -ENOMEDIUM.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>