start-instance: fix boot time measurement
start-instance: fix boot time measurement

We need to do now - start, not start - now, otherwise we get a negative
duration.

travis-ci/worker,Acidburn0zzz/worker,travis-ci/worker,solarce/worker,solarce/worker,Acidburn0zzz/worker,travis-ci/worker,Acidburn0zzz/worker,henrikhodne-abandoned/worker,solarce/worker,henrikhodne-abandoned/worker,henrikhodne-abandoned/worker