Refactor: libcrmcommon: Use pcmk__output_t in pcmk_log_xml_impl()

Add a logger_out object to logging.c; initialize it and pass it to
pcmk__xml_show() the first time do_crm_log_xml() logs something; and
free it in crm_exit() just before qb_log_fini() frees the libqb logging
objects.

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