core: disable illegal operations on a pooled VM

When a disk is defined as ReadOnly or Wipe After Delete in a VmTemplate,
after which a VmPool was created, these values shouldn't be allowed to be
changed as long as the vm is still attached to the pool.
This patch enforces this in the backend.

Change-Id: Ie3a6e1542f3ef4fb78f074b62e854a4907dfbca6
Bug-Url: https://bugzilla.redhat.com/1057721
Signed-off-by: Vered Volansky <vvolansk@redhat.com>