packaging: setup: domain validation changes

1. perform reverse lookup only if AIO plugin is installed, as it is not
   required unless libvirt is trying to perform migration.

2. allow names without suffix, warn in this case.

3. split out hostname logic from protocols.

Change-Id: I7276d3e9088fd370458846bfdcee7166331935e4
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>