schema: wrap fsDriver in a choice group

Allow adding new groups without changing indentation.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NPeter Krempa <pkrempa@redhat.com>
Acked-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Tested-by: NAndrea Bolognani <abologna@redhat.com>

docs/schemas/domaincommon.rng

@@ -2620,6 +2620,8 @@
            for this kind of info, and 'type' for the
            storage format. We need the latter too, so
            had to invent a new attribute name -->
+      <choice>
+        <group>
           <optional>
             <attribute name="type">
               <choice>
@@ -2642,7 +2644,9 @@
             </attribute>
           </optional>
           <ref name='virtioOptions'/>
+        </group>
         <empty/>
+      </choice>
     </element>
   </define>