cros_au_test_harness: Fix running VMTest serially

Fix issue that VMTest is running in parallel even if --parallel
is not set.

BUG=None
TEST=tryjob

Change-Id: Ic2095c52bd6dc8eb8a93bc4ac5a49cf7f771250e
Reviewed-on: https://chromium-review.googlesource.com/627260
Commit-Ready: Po-Hsien Wang <pwang@chromium.org>
Tested-by: Po-Hsien Wang <pwang@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
Reviewed-by: Daniel Wang <wonderfly@google.com>
2 files changed