Right now it wrongly seems as if you can set
`sshBackdoor.enable = true;` for each test and not only for debugging
purposes.
This is wrong however since you'd need to pass /dev/vhost-vsock into the
sandbox for this (which is also a prerequisite for #392117).
To make that clear, two things were changed:
* add a warning to the manual to communicate this.
* exit both interactive and non-interactive driver early if
/dev/vhost-vsock is missing and the ssh backdoor is enabled.
If that's the case, we pass a CLI flag to the driver already in the
interactive case. This change also sets the flag for the
non-interactive case.
That way we also get a better error if somebody tries to enable this
on a system that doesn't support that.
c6978e8a58