Update the mk-vmimage.sh script:

- Use mkimg(1) to create the VMDK and VHD formats.
- Skip qcow2 if qemu-img does not exist.
- Use qemu-img for VHD and VMDK if mkimg(1) does not
  exist.

Sponsored by:	The FreeBSD Foundation
Prevent stale devfs mount that is needed when building the
qemu-img qcow2 vm image.

Sponsored by:	The FreeBSD Foundation