A bdrv_drain operation must ensure that all parents are quiesced, this
includes BlockBackends. Otherwise, callbacks called by requests that are
completed on the BDS layer, but not quite yet on the BlockBackend layer
could still create new requests.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>