tests: vpp_memif fix formatting error in exception

Type: test

Change-Id: I2d32797efd1c3478a862b7950ef9ab63428da890
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

src/plugins/memif/test/vpp_memif.py

@@ -90,14 +90,11 @@ class VppMemif(VppObject):
             ring_size=self.ring_size,
             buffer_size=self.buffer_size,
             hw_addr=self.hw_addr)
-        try:
-            self.sw_if_index = 0
-        except AttributeError:
-            raise AttributeError('self: %s' % self.__dict__)
         try:
             self.sw_if_index = rv.sw_if_index
         except AttributeError:
-            raise AttributeError("%s %s", self, rv)
+            # rv doesn't have .sw_if_index attribute
+            raise AttributeError("%s %s" % (self, rv))
 
         return self.sw_if_index