scripts/gen-vbox-ova.sh: Add --vga option to vdi image

Generate osv.vdi image from 'make osv.vdi' Makefile target which will
convert to the image to vdi as well as add the --vga option.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>