qapi: ensure stable sort ordering when checking QAPI entities

Some early python 3.x versions will have different default
ordering when calling the 'values()' method on a dict, compared
to python 2.x and later 3.x versions. Explicitly sort the items
to get a stable ordering.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180116134217.8725-8-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>