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>

docs/formatnetwork.html.in

@@ -276,6 +276,20 @@
     </nat>
   </forward>
 ...</pre>
+
+            <p>
+              <span class="since">Since 6.5.0</span> it is possible to
+              enable NAT with IPv6 networking. As noted above, IPv6
+              has historically done plain forwarding and thus to avoid
+              breaking historical compatibility, IPv6 NAT must be
+              explicitly requested.
+            </p>
+            <pre>
+...
+  &lt;forward mode='nat'&gt;
+    &lt;nat ipv6='yes'/&gt;
+  &lt;/forward&gt;
+...</pre>
           </dd>
 
           <dt><code>route</code></dt>

docs/schemas/network.rng

@@ -181,6 +181,11 @@
               </optional>
               <optional>
                 <element name='nat'>
+                  <optional>
+                    <attribute name="ipv6">
+                      <ref name="virYesNo"/>
+                    </attribute>
+                  </optional>
                   <interleave>
                     <optional>
                       <element name='address'>