qapi: Fix code generation with Python 3.5

qapi: Fix code generation with Python 3.5

Recent commit 3e7fb5811b "qapi: Fix code generation for empty modules"
modules" switched QAPISchema.visit() from

    for entity in self._entity_list:

effectively to

    for mod in self._module_dict.values():
        for entity in mod._entity_list:

Visits in the same order as long as .values() is in insertion order.
That's the case only for Python 3.6 and later.  Before, it's in some
arbitrary order, which results in broken generated code.

Fix by making self._module_dict an OrderedDict rather than a dict.

Fixes: 3e7fb5811baab213dcc7149c3aa69442d683c26c
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Tested-by: Thomas Huth <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: BALATON Zoltan <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Message-id: 20200116202558[email protected]
Signed-off-by: Peter Maydell <[email protected]>