redirdev: allows to specify device address

It is important to be able to attach USB redirected devices to a
particular controller (one that supports USB2 for instance).
Without this patch, only the default bus was used.

     <redirdev bus='usb' type='spicevmc'>
       <address type='usb' bus='0' port='4'/>
     </redirdev>

docs/schemas/domaincommon.rng

@@ -2008,6 +2008,9 @@
         <ref name="qemucdevSrcTypeChoice"/>
       </attribute>
       <ref name="qemucdevSrcDef"/>
+      <optional>
+        <ref name="address"/>
+      </optional>
     </element>
   </define>
   <define name="hostdev">