Little fixes to make more of the stats reset correctly.

Little fixes to make more of the stats reset correctly.

base/statistics.cc:
    formatting
cpu/simple_cpu/simple_cpu.cc:
cpu/simple_cpu/simple_cpu.hh:
    Make numInsts reset by adding a resetStats function
sim/sim_object.cc:
    Register the reset callback in a slightly cleaner way to avoid
    potential static member constructor ordering issues

--HG--
extra : convert_revision : 408073b4b0397fbf9dfd9c548a313f1c8c3fc031