libxl_json: allow basic JSON type objects generation

libxl_json: allow basic JSON type objects generation

The original logic is that basic JSON types (number, string and null)
must be an element of JSON map or array. This assumption doesn't hold
true anymore when we need to return basic JSON types.

Returning basic JSON types is required for parsing number, string and
null objects back into libxl__json_object.

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