Fix devfs(5) mount path in create_vmimage_qemu().

Sponsored by:	The FreeBSD Foundation
Check if mkimg(1) reports back a version which can be used
to determine which disk formats can be created.  If the
version is not empty, use create_vmimage_mkimg() to create
the qcow2 format.

While here, use mkimg(1) instead of qemu-img to create the
raw disk format.

Many thanks to marcel@ for adding VHD and QCOW2 formats.

Sponsored by:	The FreeBSD Foundation