core: Prevent operations on the hosted engine disk

Prevent the hosted engine disk from being removed, attached to a VM or
updated

Change-Id: I75c1deee655278a8f575814aea22965be2bfee55
Bug-Url: https://bugzilla.redhat.com/1167668
Signed-off-by: Tal Nisan <tnisan@redhat.com>