core: fail deletion of template/template disk with derived disks

*When a template disk has derived disks on the selected domain for
deletion, the deletion should fail with a proper message.

*When deleting a template with derived disks, the deletion should fail
with proper message.

Change-Id: Ifca48993f40f3c5f7b603f33add7049e78e6c4e9
Bug-Url: https://bugzilla.redhat.com/953492
Signed-off-by: Liron Aravot <laravot@redhat.com>