librbd: disallow migrating when image has group/trash snapshot

librbd: disallow migrating when image has group/trash snapshot

Fixes:
Two potential problems with group/trash snapshot:
(1) ENOENT is returned when retrieving the snapshot status;
(2) if ignore ENOENT from (1) and allow migrating, it will failed too
when removing src image in the `migration commit` step.
So disallowing migration directly.
```
[swb@ ~/ceph/ceph-dev/build]$ rbd snap rm img1@snap1
Removing snap: 100% complete...done.
[swb@ ~/ceph/ceph-dev/build]$ rbd snap ls -a img1
SNAPID NAME                                 SIZE   PROTECTED TIMESTAMP                NAMESPACE
     4 cfeca8c8-237e-4682-b0b8-2875fd17251d 10 MiB           Thu Jan  3 15:58:59 2019 trash (snap1)
[swb@ ~/ceph/ceph-dev/build]$ rbd migration prepare img1 m1
2019-01-03 16:00:09.374 7fa758b1d040 -1 librbd::Migration: list_snaps: failed retrieving snapshot status: (2) No such file or directory
rbd: preparing migration failed: (2) No such file or directory
```

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