Correct a policy test case.

BUG=chromium:879737
TEST=This is a test case.

Change-Id: I57c11e9ad663081ec6ee56b5dd4afa58db470384
Reviewed-on: https://chromium-review.googlesource.com/1211113Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Commit-Queue: Tim Zheng <timzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589347}

chrome/test/data/policy/policy_test_cases.json

@@ -3825,10 +3825,10 @@
   "VirtualMachinesAllowed": {
     "os": ["chromeos"],
     "test_policy": {
-      "virtual_machines_allowed": false
+      "virtual_machines_allowed": true
     },
     "pref_mappings": [
-      { "pref": "cros.virtual_machines_allowed"}
+      { "pref": "cros.device.virtual_machines_allowed"}
     ]
   },