systemd: Apply VirtualBox fix for detect-virt.

The update is basically just one additional commit, which was an
upstream cherry-pick pushed at NixOS/systemd#3 and it fixes
systemd-detect-virt with VirtualBox so that services with
ConditionVirtualization set to "oracle" will work properly.

I've tested this with the "virtualbox" NixOS VM test, which was failing
since the update to version 228.

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