pyloxi: support OXM member in set-field actions

The C backend needs more work to support of_oxm_t, so I've added a hack so it
can keep using of_octets_t.

Also added several set-field action testcases.
diff --git a/test_data/of13/action_output.data b/test_data/of13/action_output.data
new file mode 100644
index 0000000..7cd52ce
--- /dev/null
+++ b/test_data/of13/action_output.data
@@ -0,0 +1,8 @@
+-- binary
+00 00 # type
+00 10 # length
+00 00 00 32 # port
+ff ff # max_len
+00 00 00 00 00 00 # pad
+-- python
+ofp.action.output(port=50, max_len=65535)
diff --git a/test_data/of13/action_set_field_eth_dst.data b/test_data/of13/action_set_field_eth_dst.data
new file mode 100644
index 0000000..1e4a971
--- /dev/null
+++ b/test_data/of13/action_set_field_eth_dst.data
@@ -0,0 +1,8 @@
+-- binary
+00 19 # type
+00 10 # length
+80 00 06 06 # OXM header
+00 01 02 03 04 05 # OXM value
+00 00 # pad
+-- python
+ofp.action.set_field(field=ofp.oxm.eth_dst([0, 1, 2, 3, 4, 5]))
diff --git a/test_data/of13/action_set_field_ipv6_src.data b/test_data/of13/action_set_field_ipv6_src.data
new file mode 100644
index 0000000..f440dee
--- /dev/null
+++ b/test_data/of13/action_set_field_ipv6_src.data
@@ -0,0 +1,7 @@
+-- binary
+00 19 # type
+00 18 # length
+80 00 34 10 # OXM header
+00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f # OXM value
+-- python
+ofp.action.set_field(field=ofp.oxm.ipv6_src("\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f"))
diff --git a/test_data/of13/action_set_field_tcp_src.data b/test_data/of13/action_set_field_tcp_src.data
new file mode 100644
index 0000000..a69c7c0
--- /dev/null
+++ b/test_data/of13/action_set_field_tcp_src.data
@@ -0,0 +1,8 @@
+-- binary
+00 19 # type
+00 10 # length
+80 00 1a 02 # OXM header
+00 32 # OXM value
+00 00 00 00 00 00 # pad
+-- python
+ofp.action.set_field(field=ofp.oxm.tcp_src(50))