vkernel - Improve memory image file startup

vkernel - Improve memory image file startup

* Remove the code that pre-filled a memory image file with zeros.  It's
  completely worthless, particularly with HAMMER.

* On startup truncate the memory file to 0 and then extend to the
  memory size, deleting any backing store from the prior vkernel run.
  The new file will start out full of holes.

  This greatly improves vkernel startup time.