Tweak USB hostdevice XML handling

When attaching a USB host device based on vendor/product, libvirt
will resolve the vendor/product into a device/bus pair. This means
that when printing XML we should allow device/bus info to be printed
at any time if present

* src/conf/domain_conf.c, docs/schemas/domain.rng: Allow USB device
  bus info alongside vendor/product

docs/schemas/domain.rng

@@ -1179,7 +1179,12 @@
       <group>
         <element name="source">
           <choice>
-            <ref name="usbproduct"/>
+	    <group>
+              <ref name="usbproduct"/>
+	      <optional>
+		<ref name="usbaddress"/>
+	      </optional>
+	    </group>
             <ref name="usbaddress"/>
             <element name="address">
               <ref name="pciaddress"/>