libxl IDL: generate code to parse libxl__json_object to libxl_FOO struct

libxl IDL: generate code to parse libxl__json_object to libxl_FOO struct

libxl_FOO_parse_json functions are generated.

Note that these functions are used to parse libxl__json_object to
libxl__FOO struct. They don't consume JSON string.

The new function definitions are generated to new header files called
__libxl_types_*_private.h so that they don't contaminate public header.
The suffix "private is chosen so we can avoid clashing with
libxl_types_internal.idl stuffs.

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