docs: update domain schema for machine attribute

Replace the long dead 'xenner' with 'xenfv'.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: NJim Fehlig <jfehlig@suse.com>

docs/schemas/domaincommon.rng

@@ -340,7 +340,7 @@
         <attribute name="machine">
           <choice>
             <value>xenpv</value>
-            <value>xenner</value>
+            <value>xenfv</value>
           </choice>
         </attribute>
       </optional>