core: Add a validation when deactivate ISO domain.

Adding a CDA when deactivating an ISO storage domain if there
are VMs with an ISO file attached to them and are not down.

Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/969641
Change-Id: I47c1a8155762ecd0b04bb17676151946982bb919