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.)