openflow_input: add bsn_gentable extension and tests

See the documentation in openflow_input/bsn_gentable.
diff --git a/test_data/of13/bsn_tlv_port.data b/test_data/of13/bsn_tlv_port.data
new file mode 100644
index 0000000..303b0ec
--- /dev/null
+++ b/test_data/of13/bsn_tlv_port.data
@@ -0,0 +1,6 @@
+-- binary
+00 00 # type
+00 08 # length
+00 00 00 05 # value
+-- python
+ofp.bsn_tlv.port(5)