libxl_json: introduce libxl__object_from_json

libxl_json: introduce libxl__object_from_json

Given a JSON string, we need to convert it to libxl_FOO struct.

The approach is:
JSON string -> libxl__json_object -> libxl_FOO struct

With this approach we can make use of libxl's infrastructure to do the
first half (JSON string -> libxl__json_object).

Second half is done by auto-generated code by libxl's IDL
infrastructure. IDL patch(es) will come later.

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