conf: fix formatting of udp chardev attributes

It is possible (although possibly not very useful) to leave out
the service attribute when using <source mode='bind'/>

Fix the formatter bug introduced by commit 4a0da34 and format
the host when its present (checked for non-NULL inside
virBufferEscapeString) instead of basing it on the presence
of the service attribute.

https://bugzilla.redhat.com/show_bug.cgi?id=1455825

tests/genericxml2xmloutdata/generic-chardev-udp.xml

@@ -29,7 +29,7 @@
       <target type='virtio' name='test3'/>
     </channel>
     <channel type='udp'>
-      <source mode='bind'/>
+      <source mode='bind' host='localhost'/>
       <source mode='connect' service='5678'/>
       <target type='virtio' name='test4'/>
     </channel>