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/basic.py b/tests/basic.py
index 66e0355..5312c1a 100644
--- a/tests/basic.py
+++ b/tests/basic.py
@@ -169,7 +169,8 @@
                logging.info("PKT OUT test with %s, port %s" % (opt, dp_port))
                msg = ofp.message.packet_out(in_port=ofp.OFPP_NONE,
                                         data=str(outpkt),
-                                        actions=[ofp.action.output(port=dp_port)])
+                                        actions=[ofp.action.output(port=dp_port)],
+                                        buffer_id=0xffffffff)
 
                logging.info("PacketOut to: " + str(dp_port))
                self.controller.message_send(msg)
@@ -222,7 +223,8 @@
                actions = [ofp.action.output(port=port) for port in dp_ports]
                msg = ofp.message.packet_out(in_port=ofp.OFPP_NONE,
                                         data=str(outpkt),
-                                        actions=actions)
+                                        actions=actions,
+                                        buffer_id=0xffffffff)
 
                logging.info("PacketOut to: " + str(dp_ports))
                self.controller.message_send(msg)