VPP-1508: Tests: Fix vpp_api struct.error under py3.

Fix struct.error: expected bytes object got <class 'str'>

Change-Id: I837ae6e97e44c789a9372677151b157956525334
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/vpp_memif.py

@@ -1,5 +1,7 @@
 import socket
 
+import six
+
 from vpp_object import VppObject
 
 
@@ -50,8 +52,9 @@ class VppSocketFilename(VppObject):
         rv = self._test.vapi.memif_socket_filename_add_del(
             1, self.socket_id, self.socket_filename)
         if self.add_default_folder:
-            self.socket_filename = self._test.tempdir + "/" \
-                                   + self.socket_filename
+            self.socket_filename = b"%s/%s" % (
+                six.ensure_binary(self._test.tempdir, encoding='utf-8'),
+                self.socket_filename)
         return rv
 
     def remove_vpp_config(self):