core: avoid broken status on snapshot removal

RemoveSnapshotCommand:
on failure, instead of declaring the entire snapshot as broken,
removing the successfully deleted disks from the snapshot
while keeping the others.

Change-Id: I2a15034185ef30f53fbeaa46440cd92954d39eee
Bug-Url: https://bugzilla.redhat.com/1056935
Signed-off-by: Daniel Erez <derez@redhat.com>