Turn on vm_tests for firmware release

Test to see if this is a workaround for failing Archive step

BUG=chrome-os-partner:9958
BUG=chromium-os:31658
TEST=cbuildbot_config_unittest

Change-Id: I16292ae508906da25b9a573d4cad4444733ab9c1
Reviewed-on: https://gerrit.chromium.org/gerrit/24900
Reviewed-by: Eric Blake <eblake@chromium.org>
Tested-by: Eric Blake <eblake@chromium.org>
diff --git a/buildbot/cbuildbot_config.py b/buildbot/cbuildbot_config.py
index 1e7fb0e..e8fce8c 100755
--- a/buildbot/cbuildbot_config.py
+++ b/buildbot/cbuildbot_config.py
@@ -957,7 +957,7 @@
   overlays=constants.BOTH_OVERLAYS,
   build_tests=True,
   unittests=True,
-  vm_tests=None,
+  vm_tests=constants.SIMPLE_AU_TEST_TYPE,
   prebuilts=False,
 )