vkernel - Add COW image capability

vkernel - Add COW image capability

* Add a copy-on-write disk image feature.  This allows a vkernel
  to mount a disk image RO or R+W but does not try to write changes
  back to the image file.

  This allows multiple vkernel instances to use the same image
  file.

* Note that when the vkernel operates on an image in this mode,
  modifications will eat up system memory and swap, so the user
  should be cognizant of the use-case.  Still, the flexiblity of
  being able to mount the image R+W should not be underestimated.