core: Make sure disk alias will never be null

In case of running a VM with disks imported from version lower than V3.0
OVFs or from P2V/V2V, the disk aliases created can be null, when the
CreateVmVdsCommand is running it sorts the disks by the aliases yet the
comparator does not check if one of the aliases is null and fails on an
NPE, this patch fixes this issue by making sure that the disk alias
property will never be null

Change-Id: I110f40b121a0bff8117888399fd5b798a9f19c10
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-url: https://bugzilla.redhat.com/1059403