conf: add an attribute to turn on NAT for IPv6 virtual networks

conf: add an attribute to turn on NAT for IPv6 virtual networks

Historically IPv6 did not support NAT, so when IPv6 was added to
libvirt's virtual networks, when requesting <forward mode="nat"/>
libvirt will NOT apply NAT to IPv6 traffic, only IPv4 traffic.

This is an annoying historical design decision as it means we
cannot enable IPv6 automatically. We thus need to introduce a
new attribute

   <forward mode="nat">
     <nat ipv6="yes"/>
   </forward>

Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>