add "stable" parameter to enums

This parameter gives a hint to the backend that the enum members have the same
value in all OpenFlow versions. It's enforced when generating the unified IR.

The stable parameter is added to the BSN port/vlan/vrf stats extensions.
diff --git a/loxi_ir/unified.py b/loxi_ir/unified.py
index 41b86dc..fa97c87 100644
--- a/loxi_ir/unified.py
+++ b/loxi_ir/unified.py
@@ -98,6 +98,9 @@
                     self.entries[e.name] = ir.OFEnumEntry(e.name, e.value, copy.copy(e.params))
                 else:
                     entry = self.entries[e.name]
+                    if v_enum.params.get('stable') == 'True' and e.value != entry.value:
+                        raise Exception("Error unifying stable ir enum {} - adding entry {} version {} value {} <-> {}".format(
+                            self.name, e.name, version, entry.value, e.value))
                     for name, value in e.params.items():
                         if not name in entry.params:
                             entry.params[name] = value