util: add support for IPv6 masquerade rules
util: add support for IPv6 masquerade rules

IPv6 does support masquerade since Linux 3.9.0 / ip6tables 1.4.18,
which is Fedora 18 / RHEL-7 vintage, which covers all our supported
Linux versions.

Reviewed-by: Laine Stump <c23361c43fbf79fed83e8b76173707b083d6caf5@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

crobinso/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,olafhering/libvirt,jardasgit/libvirt,crobinso/libvirt,jfehlig/libvirt,crobinso/libvirt,nertpinx/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt