Add UUID definition required by storage encryption import

* docs/schemas/storagevol.rng: storage encryption rules require
  that UUID type be defined

docs/schemas/storagevol.rng

@@ -198,5 +198,15 @@
     </data>
   </define>
 
+  <define name="UUID">
+    <choice>
+      <data type="string">
+        <param name="pattern">[a-fA-F0-9]{32}</param>
+      </data>
+      <data type="string">
+        <param name="pattern">[a-fA-F0-9]{8}\-([a-fA-F0-9]{4}\-){3}[a-fA-F0-9]{12}</param>
+      </data>
+    </choice>
+  </define>
 
 </grammar>