core: Fix Live Merge regression

A change in CoCo exposed some bad code in DestroyImageCommand; fix the
async task creation to not reference the wrong command type.

Change-Id: I90e4b96cb673975c964fcb37146ad6baa510c8eb
Bug-Url: https://bugzilla.redhat.com/1141985
Signed-off-by: Greg Padgett <gpadgett@redhat.com>