Allow custom metadata in domain configuration XML

Applications can now insert custom nodes and hierarchies into domain
configuration XML. Although currently not enforced, applications are
required to use their own namespaces on every custom node they insert,
with only one top-level element per namespace.

docs/formatdomain.html.in

@@ -3556,6 +3556,26 @@ qemu-kvm -net nic,model=? /dev/null
       sub-element <code>label</code> are supported.
     </p>
 
+    <h3><a name="customMetadata">Custom metadata</a></h3>
+
+<pre>
+  ...
+  &lt;metadata&gt;
+    &lt;app1:foo xmlns:app1="http://app1.org/app1/"&gt;..&lt;/app1:foo&gt;
+    &lt;app2:bar xmlns:app2="http://app1.org/app2/"&gt;..&lt;/app2:bar&gt;
+  &lt;/metadata&gt;
+  ...</pre>
+
+    <dl>
+      <dt><code>metadata</code></dt>
+      <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 0.9.10</span></dd>
+    </dl>
+
     <h2><a name="examples">Example configs</a></h2>
 
     <p>

docs/schemas/domaincommon.rng

@@ -25,6 +25,9 @@
         <optional>
           <ref name="description"/>
         </optional>
+        <optional>
+          <ref name="metadata"/>
+        </optional>
         <optional>
           <ref name="cpu"/>
         </optional>
@@ -2942,6 +2945,29 @@
     </element>
   </define>
 
+  <define name="metadata">
+    <element name="metadata">
+      <zeroOrMore>
+        <ref name="customElement"/>
+      </zeroOrMore>
+    </element>
+  </define>
+
+  <define name="customElement">
+    <element>
+      <anyName/>
+      <zeroOrMore>
+        <choice>
+          <attribute>
+            <anyName/>
+          </attribute>
+          <text/>
+          <ref name="customElement"/>
+        </choice>
+      </zeroOrMore>
+    </element>
+  </define>
+
   <!--
        Type library