core: Fix event message when removing a LUN disk

When removing a disk, the event message included the domain name parameter
in case the disk is a LUN disk there is no storage domain and the message
appeared incorrectly, added a new type of event message for LUN disks that
displays the same message without the storage domain parameter

Change-Id: I7624619d27e9e9fd6ffdc09f4380b9d2dbe5624b
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-url: https://bugzilla.redhat.com/910733