Refactor: various: Match elements directly with get_xpath_object()

Now all internal callers call get_xpath_object() with XPath expressions
that can only match elements. This avoids the need to rely on the
strange pcmk__xpath_match_element() logic when we replace
get_xpath_object() with an internal function.

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