Refactor: libcrmcommon: New pcmk__xpath_find_one()

To replace get_xpath_object().

get_xpath_object() dereferences its xmlNode argument without
NULL-checking it. So the fact that we now dereference it in all of the
callers to get its doc member, shouldn't cause any NEW problems. Later,
in per-function best practices commits, we should NULL-check the
arguments where appropriate.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>