https://bugzilla.redhat.com/show_bug.cgi?id=1515533

We're already checking if IPv4 prefix isn't too long. But we are
not checking if it isn't too short. QEMU supports prefixes longer
than 4 (including). I haven't find anything similar related to
IPv6 in qemu sources.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>