iotests/124: Add cluster_size mismatch test

If a backing file isn't specified in the target image and the
cluster_size is larger than the bitmap granularity, we run the risk of
creating bitmaps with allocated clusters but empty/no data which will
prevent the proper reading of the backup in the future.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1456433911-24718-4-git-send-email-jsnow@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>