Simplify image probing

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
vbox: Fail if image format is not supported

Fail if the user attempts to run an unsupported image format under
VirtualBox rather than starting up a VM that just hangs.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>