core: export vm/create template shouldn't include ILLEGAL disks

When exporting a VM/Creating a template - return CDA message when there
are image disks which are in ILLEGAL status, this prevents the following:
*additional vdsm commands
*possible status change from ILLEGAL to ok
*failure of the entire operation

Change-Id: I6e608c0c87c6e3a8d275fd74658b888cde6c8694
Bug-Url: https://bugzilla.redhat.com/921489
Signed-off-by: Liron Aravot <laravot@redhat.com>