core: Template's empty string as a disk alias fix.

While making a new template based on a VM, providing the template's
allocated disks' alias as an empty string is now disabled. The user
should not be able to create a template with disks that have an
empty alias.

Change-Id: If20fcbe3ae6507ba1f39c6e336928fb647efeb1e
Bug-Url: https://bugzilla.redhat.com/1110304
Signed-off-by: Amit Aviram <aaviram@redhat.com>