vkernel - Print mmap() failures in megabytes

* Print mmap() failures for vkd* drives in megabytes instead
of bytes.

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.

T

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.

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