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/openflow_input/standard-1.3 b/openflow_input/standard-1.3
index ccef4db..0d5cec3 100644
--- a/openflow_input/standard-1.3
+++ b/openflow_input/standard-1.3
@@ -824,7 +824,7 @@
 struct of_action_set_field {
     uint16_t type == 25;
     uint16_t len;
-    of_octets_t field;
+    of_oxm_t field;
 };
 
 struct of_action_experimenter {