Make it so that we create a directory for each checkpoint so that

Make it so that we create a directory for each checkpoint so that
there aren't so many files littering your directory

dev/disk_image.cc:
    Checkpoints now in a directory
sim/serialize.hh:
    Make it so that we create a directory for each checkpoint so that
    there aren't so many files littering your directory.
    Remove unused variable

--HG--
extra : convert_revision : 261824eee62f7b68f6ae6e3dbd49ad5128ced148