Refactor: libs: Set XML properties when creating a node.
    
    pcmk__output_create_xml_node can take a NULL-terminated list of
    properties that should be set on the node at the same time it's created.
    This saves a bunch of calls to xmlSetProp and gets rid of some variables
    that are otherwise not needed.
    
    Unfortunately, this can't be used everywhere I'd like but it's a good
    start.