systemd-detect-virt: only discover Xen domU

The current vm detection lacks the distinction between Xen dom0 and Xen domU.
Both, dom0 and domU are running inside the hypervisor.
Therefore systemd-detect-virt and the ConditionVirtualization directive detect
dom0 as a virtual machine.

dom0 is not using virtual devices but is accessing the real hardware.
Therefore dom0 should be considered the virtualisation host and not a virtual
machine.

https://bugs.freedesktop.org/show_bug.cgi?id=77271