Refactor: libcrmcommon: Add more XML functions.

create_xml_text_node is similar to create_xml_node, but also adds text
content to the new node. This just saves callers a step and makes using
create_xml_node more appealing in more places.

pcmkoutput_xml_node is similar to create_xml_node, but is useful for
formatted output funtions. Instead of taking a parent node, it takes a
pcmkoutput_t and adds the new node as a child of whatever the current
list parent is. This is necessary to prevent leaking private data out
of the XML formatted output code.

And then, use these functions throughout.