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.