core: Live snapshot should order child commands

End method of live snapshot should order the list of the child
commands by image id to avoid dead lock with UpdateRunTimeInfo when updating
disk image dynamic.

Change-Id: I2999a2f9fb042a70eaf34e17668877040d57a068
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/909937