Reconcile VirtualBox status constants with VBoxManage output values

While looking into an issue I was having with the VirtualBox executor, I ended up updating its `StatusType` constants to reflect the strings that the VBoxManage utility provides when running the `showvminfo` command. A couple parts of `executor_virtualbox.go` were affected by this change, which I also updated accordingly in this branch.

(For reference, the specific issue I was running into was that the `Kill()` function in `helpers/virtualbox/control.go` would return before the VM could finish shutting down since `acpipowerbutton` only sends a signal to the guest OS, which was in turn causing the next job to fail to restore the VM snapshot since it was still running. I have a quick fix in my `fix-virtualbox-kill` branch, but it relies on one of the `StatusType` constants in this change.)

See merge request !103