librbd: disallow migrating a parent image

librbd: disallow migrating a parent image

Fixes:
```
[swb@ ~/ceph/ceph-dev/build]$ rbd snap create img2@snap1
[swb@ ~/ceph/ceph-dev/build]$ rbd clone img2@snap1 clone1 --rbd-default-clone-format 2
[swb@ ~/ceph/ceph-dev/build]$ rbd migration prepare img2 m2
[swb@ ~/ceph/ceph-dev/build]$ rbd migration execute m2
Image migration: 100% complete...done.
[swb@ ~/ceph/ceph-dev/build]$ rbd migration commit m2
2019-01-03 09:55:39.914 7fa5e177a040 -1 librbd::image::RemoveRequest: 0x560084be6e30 check_image_snaps: image has snapshots - not removing
2019-01-03 09:55:39.989 7fa5e177a040 -1 librbd::Migration: remove_src_image: failed removing source image: (39) Directory not empty
Commit image migration: 0% complete...failed.
rbd: committing migration failed: (39) Directory not empty
```

Signed-off-by: songweibin <song.weibin@zte.com.cn>