- Switch events to use a priority enum instead of integers.

- Switch events to use a priority enum instead of integers.
This lets us centralize priorities so we can see what's going on.
- Shift serialize & cpu-switch events to happen before CPU ticks (to be
consistent with starting new CPU on same cycle instead of next cycle).
- Get rid of unnecessary bus stats reset callback.

cpu/simple_cpu/simple_cpu.cc:
sim/debug.cc:
sim/eventq.hh:
sim/serialize.cc:
sim/sim_events.cc:
sim/sim_events.hh:
    Switch events to use a priority enum instead of integers.
    This lets us centralize priorities so we can see what's going on.

--HG--
extra : convert_revision : 510d79b43c0a1c97a10eb65916f7335b1de8b956