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