cpu: Support trace termination in BaseTrafficGen

cpu: Support trace termination in BaseTrafficGen

Make the BaseTrafficGen handle cases where getNextPacket() can't find
a new packet and returns NULL. In that case, assume the generator has
run out of packets and switch to the next generator.

Change-Id: I5ca6ead550005812fb849ed9ce6b5007a65ddfa7
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11517
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>