devmode-test: Fix cleanup failure when VM setup fails

VM setup can fail while preparing the test, leaving the vm attribute
None. Deal with it.

BUG=chromium:777461
BUG=chromium:770764
TEST=None

Change-Id: Id189343cf4f21cacb702507c0a6b9c249e0eaf30
Reviewed-on: https://chromium-review.googlesource.com/741684
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
1 file changed