pyloxi: remove automatic conversion from str to OFReader

User code shouldn't call these methods directly.
diff --git a/py_gen/tests/of12.py b/py_gen/tests/of12.py
index fcdb0cf..4774672 100644
--- a/py_gen/tests/of12.py
+++ b/py_gen/tests/of12.py
@@ -31,6 +31,7 @@
 try:
     import loxi
     import loxi.of12 as ofp
+    from loxi.generic_util import OFReader
 except ImportError:
     exit("loxi package not found. Try setting PYTHONPATH.")
 
@@ -89,7 +90,7 @@
                 obj = klass()
                 if hasattr(obj, "xid"): obj.xid = 42
                 buf = obj.pack()
-                obj2 = klass.unpack(buf)
+                obj2 = klass.unpack(OFReader(buf))
                 self.assertEquals(obj, obj2)
             if klass in expected_failures:
                 self.assertRaises(Exception, fn)