systemd: Backport fix for detecting VirtualBox.

This is a backport of systemd/systemd@e32886e.

As noted by @ts468 in #9876, systemd-detect-virt will report KVM if
we're running inside VirtualBox 5.x. Instead of just disabling the
check, this essentially fixes systemd to be able to detect VirtualBox
again.

Tested this against nixos/tests/simple.nix (just to make sure systemd is
still working) and nixos/tests/virtualbox.nix (all tests succeed).

Thanks a lot to @ts468 for catching this and also to @domenkozar for
testing various things concerning that bug.

Fixes #9876.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>