misc: Clean up and complete the gem5<->SystemC-TLM bridge [10/10]

misc: Clean up and complete the gem5<->SystemC-TLM bridge [10/10]

The current TLM bridge only provides a Slave Port that allows the gem5
world to send request to the SystemC world. This patch series refractors
and cleans up the existing code, and adds a Master Port that allows the
SystemC world to send requests to the gem5 world.

This patch:
  * Add callbacks for the Gem5SimControl that are called at before and
  * after simulate()

Reviewed at http://reviews.gem5.org/r/3799/

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>