checkpoint: generate GIR information for NMCheckpointCreateFlags

Note that this changes API for checkpoint_create_async() in Python
via GIR. Previously it would require an integer argument, now a flags
argument. But this API is still unstable, it will be introduced with
1.12.

examples/python/gi/checkpoint.py

@@ -48,6 +48,7 @@ def do_create(client):
             sys.exit("Unknown device %s" % arg)
         devices.append(d)
 
+    # FIXME: flags are not a plain integer.
     client.checkpoint_create_async(devices, timeout, 0, None, create_cb, None)
 
 def destroy_cb(client, result, data):