test-functions: don't nest KVM (#8883)

Nested KVM is very flaky as we learnt from our CI. Hence, let's avoid
KVM whenever we detect we are already running inside of KVM.

Maybe one day nested KVM is fixed, at which point we can turn this on
again, but for now let's simply avoid nested KVM, since reliable CI is
more important than quick CI, I guess.

And yes, avoiding KVM for our qemu runs does make things substantially
slower, but I think it's not a complete loss.

Inspired by @evverx' findings in:

https://github.com/systemd/systemd/pull/8701#issuecomment-380213302