libxlu: introduce new APIs

libxlu: introduce new APIs

These APIs can be used to manipulate XLU_ConfigValue and XLU_ConfigList.

APIs introduced:
1. xlu_cfg_value_type
2. xlu_cfg_value_get_string
3. xlu_cfg_value_get_list
4. xlu_cfg_get_listitem2

Move some definitions from private header to public header as needed.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>