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/load.py b/tests/load.py
index 4680edb..2631f36 100644
--- a/tests/load.py
+++ b/tests/load.py
@@ -79,6 +79,7 @@
         msg = ofp.message.packet_out()
         msg.in_port = lb_port
         msg.data = str(pkt)
+        msg.buffer_id = 0xffffffff
         act = ofp.action.output()
         act.port = lb_port + 1
         msg.actions.append(act)
@@ -171,6 +172,7 @@
                act = ofp.action.output()
                act.port = dp_port
                msg.actions.append(act)
+               msg.buffer_id = 0xffffffff
 
                logging.info("PacketOutLoad to: " + str(dp_port))
                for count in range(100):