In a typical over provisioned system (you can think of any folder-based filesystem as being this way too) you have many VMs, not just one or two. Once you have, say, a dozen it is trivial to over provision safely. You do this by a few means:
Reporting and alerting so that systems growing rapidly are addressed. Maintaining caps that are large enough for flexibility but small enough so that if X VMs spiral out of control it will not be an issue. The X factor here is to be determined based on your risk and risk aversion. If you have, say, a dozen VMs, you might have the cap set so that ten VMs could expand to maximum but not all twelve. If any two VMs don't have an issue, you would be all set. Or maybe you'd set it for six. You don't overprovision to where any VM could use the entire space, it just doesn't make sense.