qapi: Fix code generation with Python 3.5

Recent commit 3e7fb581 "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: 3e7fb581
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Thomas Huth <thuth@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200116202558.31473-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>