core: RemoveDiskSnapshots - sort non-consecutive images

RemoveDiskSnapshots -> initTaskHandlers:
fix images sorting in case of non consecutive images
(e.g. image_index_0, image_index_3, image_index_7).

Change-Id: Ia736aa93a76dd8df13bfdcf97587cee82bea6700
Bug-Url: https://bugzilla.redhat.com/1151841
Signed-off-by: Daniel Erez <derez@redhat.com>