Merge pull request xapi-project#5926 from contificate/is_required_param

Output if parameter is required in JSON backend

ocaml/idl/json_backend/gen_json.ml

@@ -197,6 +197,7 @@ end = struct
                 ("type", `String t)
               ; ("name", `String p.param_name)
               ; ("doc", `String p.param_doc)
+              ; ("required", `Bool (Option.is_none p.param_default))
               ]
             :: params
           , enums @ e