tests/virtualbox: Add systemd-detect-virt subtest.

Addresses #9876 in the way that we want to make sure that VirtualBox 5.x
is going to be properly detected. Right now the result is "kvm", so the
subtest fails as expected with:

error: systemd-detect-virt returned "kvm" instead of "oracle" at (eval
       14) line 414, <__ANONIO__> line 92.

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