core: Fix regression in copying template disk

Fix regression introduced by Change-Id
Id4b5ef206581e5f9a1727c85b30f39802859583c which used the disk's ID
instead of the image ID as VmTemplateDao#getAllForImage(Guid)'s
parameter.

Change-Id: I2627741221aee980534a5cd19f408d03d23a6cc4
Bug-Url: https://bugzilla.redhat.com/1006811
Signed-off-by: Allon Mureinik <amureini@redhat.com>