webadmin: Fix disk alias recycling

When creating a new disk via webadmin, the suggested disk alias created the
suggested alias from the VM name concated with the amount of the disk on the
VM + 1, in case all disk were created by that convention, deleting a disk
which is not the last one and adding a new disk resulted in a suggested disk
alias which matches the last disk alias exactly

Change-Id: Ie852fbcf26c0742c610631a121b9bfd706c01218
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-url: https://bugzilla.redhat.com/1022795