Warn users when running inside a VM

We tell users, when running inside a VM, that they are implicitly trusting both the VM software and the host OS; the warning message points to a dedicated documentation page. This is needed to avoid creating a false sense of security, which is often worse than no security and a clear sense of it.

The detection is done with systemd-detect-virt.

• config/chroot local-includes/lib/systemd/user/tails-virt-notify-user.service
• config/chroot local-includes/usr/local/lib/tails-virt-notify-user

A bit more work is left to do.