domaincaps: Report video modelType

Requires adding the plumbing for <device><video>
The value is <enum name='modelType'> to match the associated domain
XML of <video><model type='XXX'/>

Wire it up for qemu too

docs/schemas/domaincaps.rng

@@ -73,6 +73,7 @@
       <interleave>
         <ref name='disk'/>
         <ref name='graphics'/>
+        <ref name='video'/>
         <ref name='hostdev'/>
       </interleave>
     </element>
@@ -92,6 +93,13 @@
     </element>
   </define>
 
+  <define name='video'>
+    <element name='video'>
+      <ref name='supported'/>
+      <ref name='enum'/>
+    </element>
+  </define>
+
   <define name='hostdev'>
     <element name='hostdev'>
       <ref name='supported'/>