Commits 905629f4 and 1716e7a6 have added support for specifying
an IPv4 range and a port range to be used by NAT:
<forward mode='nat'>
<nat>
<address start='10.20.30.40' end='10.20.30.44'/>
<port start='60000' end='65432'/>
</nat>
</forward>
https://bugzilla.redhat.com/show_bug.cgi?id=1004364
