vmctl(8): remove unveil from disk image creation routines

vmctl(8): remove unveil from disk image creation routines

James Cook reported an issue creating images from qcow2 disks that
contained base images. Initial approach suggested was to replace the
use of realpath(3), but since this is common code used in vmd(8) the
better option is to just remove the use of unveil(1) in vmctl. (It was
added very early in vmctl's development before support for disk
conversion was added.)

This does not change existing unveil usage in vmctl for things like the
control socket or the serial console. There's no change to vmd either.

Discussed with and OK from mlarkin@