cpu-tester: Added ExitGen to TrafficGen

cpu-tester: Added ExitGen to TrafficGen

Added the ExitGen to the TrafficGenerator which allows an EXIT
state to be added to the TrafficGen configuration file. Entering this
state will cause the simulation to exit immediately. Please note that
if multiple TrafficGen instances have an EXIT state, the first of these
to be encountered will cause the simulation to terminate.

Change-Id: Ieea51f05ffb780771f007787a2b119f79143d0c1
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5723
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>