core: Change existing disk validation CDA message in case of null disk alias

When try to import a VM which one or more of it's disks already exist on
the system there's a CDA message containing the disk aliases of the existing
disks, the aliases are taken from the disks in the export domian but in case
the alias is null (mainly in backups of versions <V3.0, P2V, V2V etc..) the
alias was displayed as a blank string, the CDA message was change and in case
the alias is null the alias will be taken from the disk in the system

Change-Id: I07b794b0fe9bdc5c83a62a74b82653ce209327b7
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/928864