webadmin: Change disk alias suggestions to use alias recycling

In change 24061, disk alias suggestions was changed not to suggest a disk
alias that is already attached to the VM, this change now also makes sure
that the aliases will be recycled (i.e. if we had alias1 and alias2 and we
deleted alias1, the next suggestion will be alias1)

Change-Id: I8d5b8ba00a144b864208d971def7087182538be2
Bug-Url: https://bugzilla.redhat.com/1022795
Signed-off-by: Tal Nisan <tnisan@redhat.com>