Basically all the functions with xe in the name.

It would be most of xml.c, but the separation would help keep the distinction between any-node and elements-only fresh in our minds when working on it.

From https://github.com/ClusterLabs/pacemaker/pull/3403#discussion_r1557853912

I've got some other XML refactors pending after CLPR#3403. It will be easier to do this after merging those, rather than resolving the merge conflicts.