An update to xvfb.py changed its input arguments and the previously
required first one pointing to the execution directory has been removed.
This change adds makes it backwards-aware: if the first argument, now
expected to be the command, points to a directory it exists gracefully
with a helpful error message.

BUG=

Review-Url: https://codereview.chromium.org/2748123004
Cr-Commit-Position: refs/heads/master@{#456957}