Refactor: libraries: Call pcmk__output_xml_pop_parent.

Any custom output message that calls pcmkoutput_create_parent should
later call pcmkoutput_xml_pop_parent. Without doing this, any other
XML nodes created after the custom message will be created with an
unexpected parent.

Big caveat: The "node" custom message works this way on purpose,
however. It counts on the fact that it creates the parent and exits,
then other things happen, and then the parent is popped. I can't
convince myself that the parent is correctly being popped in every
instance but that looks like the intent.