[VirtualBox] Use systemd-detect-virt to detect if we can install

[Dominik 'Rathann' Mierzejewski]

Hi Sérgio,
could you explain the idea here? Why was there an explicit Conflicts:
before and why is it better to fail the transaction if you're not
running inside VirtualBox or KVM? Also, what if calling systemd-detect-virt
fails for some reason?

Regards,
Dominik
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