systemc: Centralize module callbacks and report new warnings.

systemc: Centralize module callbacks and report new warnings.

By centralizing module callbacks, the gem5 module class knows when
different stages of the simulation are happening and can do it's own
extra checks. It also compartmentalizes modules more since the kernel
object doesn't have to reach into them to enumerate ports and exports.

Change-Id: I55887284af9c05150fe9d054f5b6147cad6092a1
Reviewed-on: https://gem5-review.googlesource.com/c/12610
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>