misc,sim: Tagged API methods in sim/serialize.hh

misc,sim: Tagged API methods in sim/serialize.hh

Within this some light refactoring has been carried out to avoid
accessing member variable directly and removing some unused/unneeded
ones from the codebase.

Change-Id: I458494f6466628b213816c81f6a8ce42fb91dc3f
Issue-on: https://gem5.atlassian.net/browse/GEM5-172
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27989
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>