In mi-lttng[.hc] add the following functions:
	const char *enabled_string(int value);
	const char *filter_string(int value);
	const char *exclusion_string(int value);
	const char *loglevel_string(int value);
	const char *logleveltype_string(enum lttng_loglevel_type value);

They were defines in list.c but since we needed them for the mi we just
moved them.
The mi-lttng.xsd now support all the output of the mi.