Fix 'No space left on device' bug

When building a VirtualBox guest on a Windows host (and perhaps in other cases), sometimes the script won't complete because the shutdown command (echo '/sbin/halt -h -p' > shutdown.sh; echo 'vagrant'|sudo -S sh 'shutdown.sh') fails due to "write error: No space left on device".

I've fixed this error in the attached code by sleeping until the device recognizes the /EMPTY file has been deleted or 20 seconds have elapsed. In practice about 5 seconds is all that's required on my computer.