libxl: IDL: autogenerate functions to produce JSON from libxl data structures.

libxl: IDL: autogenerate functions to produce JSON from libxl data structures.

Two functions are provided. TYPE_gen_json exposes an interface which is
compatible with the YAGL generator infrastructure. TYPE_to_string uses this to
produce a pretty printed string.

The TYPE_gen_json functions are defined in a new header libxl_json.h which is
not exposed via libxl.h due to the use of YAGL datatypes to avoid poluting the
namespace us libxl users which don't use the library themselves.  If a libxl
user is interested in integrating at the YAGL level then it should #include
this file itself.

Also update testidl to generate a random version of each IDL datastructure and
convert it to JSON. Unfortunately this requires a libxl_ctx and therefore the
test must be run on a Xen system now.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>