Allow custom metadata in network configuration XML

This replicates the metadata field found in the domain configuration
    and adds it to the network configuration XML.

docs/formatnetwork.html.in

@@ -38,6 +38,10 @@
       <network ipv6='yes' trustGuestRxFilters='no'>
         <name>default</name>
         <uuid>3e3fce45-4f53-4fa7-bb32-11f34168b82b</uuid>
+        <metadata>
+          <app1:foo xmlns:app1="http://app1.org/app1/">..</app1:foo>
+          <app2:bar xmlns:app2="http://app1.org/app2/">..</app2:bar>
+        </metadata>
         ...</pre>
 
     <dl>
@@ -54,6 +58,12 @@
         The format must be RFC 4122 compliant, eg <code>3e3fce45-4f53-4fa7-bb32-11f34168b82b</code>.
         If omitted when defining/creating a new network, a random
         UUID is generated. <span class="since">Since 0.3.0</span></dd>
+      <dd>The <code>metadata</code> node can be used by applications to
+        store custom metadata in the form of XML nodes/trees. Applications
+        must use custom namespaces on their XML nodes/trees, with only
+        one top-level element per namespace (if the application needs
+        structure, they should have sub-elements to their namespace
+        element). <span class="since">Since 2.1.0</span></dd>
       <dt><code>ipv6</code></dt>
       <dd>When set to <code>yes</code>, the optional parameter
         <code>ipv6</code> enables