schema: Allow interleaving the /domain/os/type element

The element wasn't declared under the interleave thus it was required
always to be first. This made it inconvenient when pasting new stuff to
the XML manually in the "wrong" place.

docs/schemas/domaincommon.rng

@@ -238,8 +238,8 @@
       <ref name="bootloader"/>
     </optional>
     <element name="os">
-      <ref name="ostypehvm"/>
       <interleave>
+        <ref name="ostypehvm"/>
         <optional>
           <element name="loader">
             <optional>