Document that NetVMs of Windows VMs must be trusted

[DemiMarie]

How to file a helpful issue

The problem you're addressing (if any)

Windows VMs use a netfront driver that is not hardened against attacks by a malicious netback driver. Worse, #7687 means that if a Windows VM is running any of a variety of network services, any other Windows VMs based on the same template could attack it easily, needing only the ability to make a network connection to said VM.

The solution you'd like

Document that the NetVM of a Windows VM must be as trusted as the Windows VM itself, and that allowing incoming network connections to any Windows VM other than a StandaloneVM is a terrible idea.

The value to a user, and who that user might be

Users who use Windows VMs will be less likely to expose them to attack.