Use the right base path for vm updates.

bin/cros_au_test_harness.py

 #!/usr/bin/python
 
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import optparse
 import os
 import sys
 import unittest
@@ skipping 234 matching lines @@
                   '--board=%s' % board,
                   '--test_image'], enter_chroot=True)
     else:
       Info('Using existing VM image')
 
     self.assertTrue(os.path.exists(self.vm_image_path))
 
   def UpdateImage(self, image_path, stateful_change='old'):
     """Updates VM image with image_path."""
     stateful_change_flag = self.GetStatefulChangeFlag(stateful_change)
+    if self.source_image == base_image_path:
+      self.source_image = self.vm_image_path
 
     RunCommand(['%s/cros_run_vm_update' % self.crosutilsbin,
                 '--update_image_path=%s' % image_path,
                 '--vm_image_path=%s' % self.vm_image_path,
                 '--snapshot',
                 vm_graphics_flag,
                 '--persist',
                 '--kvm_pid=%s' % _KVM_PID_FILE,
                 stateful_change_flag,
                 '--src_image=%s' % self.source_image,
@@ skipping 70 matching lines @@
     else:
       remote = options.remote
 
     suite = unittest.TestLoader().loadTestsFromTestCase(RealAUTest)
     test_result = unittest.TextTestRunner(verbosity=2).run(suite)
   else:
     parser.error('Could not parse harness type %s.' % options.type)
 
   if not test_result.wasSuccessful():
     Die('Test harness was not successful')