Change type from "virtualbox" to "virtualbox-iso"

I had to edit the VirtualBox template because Packer as of version 0.5.2 has two VirtualBox types, `virtualbox-iso` and `virtualbox-ovf`. I'm guessing Hashi renamed `virtualbox` to `virtualbox-iso` but didn't leave an alias for any kind of forward/backward compatibility.

(I'm also having trouble with the shutdown command in this template, but I can report that in a separate ticket if/when I have more information.)