schema: Add virtio-mmio address type into RNG

The "virtio-mmio" is perfectly valid address type which we parse and
format correctly, but it's missing in our RNG schemas, hence editing a
domain with device having such address fails the validation.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

docs/schemas/domaincommon.rng

@@ -4415,6 +4415,11 @@
           </attribute>
           <ref name="isaaddress"/>
         </group>
+        <group>
+          <attribute name="type">
+            <value>virtio-mmio</value>
+          </attribute>
+        </group>
       </choice>
     </element>
   </define>