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

dev/disk_image.cc

@@ -405,7 +405,7 @@ CowDiskImage::write(const uint8_t *data, off_t offset)
 void
 CowDiskImage::serialize(ostream &os)
 {
-    string cowFilename = CheckpointFile() + "." + name() + ".cow";
+    string cowFilename = CheckpointDir() + name() + ".cow";
     SERIALIZE_SCALAR(cowFilename);
     save(cowFilename);
 }