tests: raise exception in vpp_interface for invalid sw_if_index

Type: test

Change-Id: I34058f2bd20f61a8f8355299a8df95f2948c25eb
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/vpp_interface.py

@@ -237,6 +237,9 @@ class VppInterface(object):
             self.sw_if_index, mac.packed)
 
     def set_sw_if_index(self, sw_if_index):
+        if sw_if_index > 255:
+            raise RuntimeError("Don't support sw_if_index values "
+                               "greater than 255.")
         self._sw_if_index = sw_if_index
 
         self.generate_remote_hosts()