Refactor: libcrmcommon: New pcmk__xe_get_ll()

To replace crm_element_value_ll().

IMO, it's better not to log errors or warnings within the getter
function. The caller has more context and can log errors or warnings if
appropriate. Sometimes, a parse failure may be expected as a legitimate
possibility.

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