BZ #1151603: Change interface filter to look at non-VIP interfaces

https://bugzilla.redhat.com/show_bug.cgi?id=1151603

The interface assignment screen was supposed to filter out already
assigned, non-vip subnets. It was changed to 'vip' in a refactor. What
then happened is that the subnet where the vips were assigned would get
filtered out, and could no longer be assigned to an interface.