Stop devtest if user was not in libvirtd group

The current mechanism for picking up the new group doesn't work
correctly anyway (see bug report for a full example of the current
behavior), and it's more friendly to stop immediately and let the
user know what they need to do instead of continuing on and
failing, which buries the relevant message further up the
scrollback.

Change-Id: I94bdb331567b92b572953d839605ed4c2108e8ec
Closes-Bug: 1285285