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/nicira_dec_ttl.py b/tests/nicira_dec_ttl.py
index f5e8214..b29f611 100644
--- a/tests/nicira_dec_ttl.py
+++ b/tests/nicira_dec_ttl.py
@@ -26,6 +26,7 @@
         outpkt = simple_tcp_packet(pktlen=100, ip_ttl=3)
         msg = ofp.message.packet_out(in_port=ofp.OFPP_NONE,
                                      data=str(outpkt),
+                                     buffer_id=x0xffffffff,
                                      actions=[
                                          ofp.action.nicira_dec_ttl(),
                                          ofp.action.output(port=portA),