cpu: Warn when (un)serializing a traffic generator

cpu: Warn when (un)serializing a traffic generator

When checkpointing a system with a traffic generator, a warning is
produced so that the user is reminded serialization does not keep all
the traffic generator internal state.

Change-Id: I3c49c912c9ff3a4208f55b2da0a88fc694147280
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11831
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>