core: fix entity type setting

When invoking RemoveImage from CreateImageTemplate, we used to set the
entity type to VM in the EntityInfo of the RemoveImage command
instead of Disk. This patch fix it.

Change-Id: I657b63fd5474be91c5eed4db876cc2942aa375ab
Signed-off-by: Arik Hadas <ahadas@redhat.com>