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 4a0da345 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