Refactor: libcrmcommon: New pcmk__xe_set_timeval()

To replace crm_xml_add_timeval().

Currently, we never check the return values of these crm_xml_add_*()
functions except internally within other crm_xml_add_*() functions. So
while a standard Pacemaker return code makes sense, there's no need to
add one to the interface unless the callers want to check it.

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