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/actions.py b/tests/actions.py
index 494d941..2791d5a 100644
--- a/tests/actions.py
+++ b/tests/actions.py
@@ -396,6 +396,7 @@
         pkt_out =ofp.message.packet_out();
         pkt_out.data = str(pkt)
         pkt_out.in_port = of_ports[0]
+        pkt_out.buffer_id = 0xffffffff
         act = ofp.action.output()
         act.port = ofp.OFPP_TABLE
         pkt_out.actions.append(act)