explictly set buffer_id for packet-out messages

The old pylibopenflow library did this by default, but PyLoxi initializes that
field to zero.
diff --git a/tests/openflow_protocol_messages.py b/tests/openflow_protocol_messages.py
index c961f2e..b30da7c 100644
--- a/tests/openflow_protocol_messages.py
+++ b/tests/openflow_protocol_messages.py
@@ -222,6 +222,7 @@
                 act = ofp.action.output()
                 act.port = dp_port
                 msg.actions.append(act)
+                msg.buffer_id = 0xffffffff
 
                 logging.info("PacketOut to: " + str(dp_port))
                 self.controller.message_send(msg)