set virtual to xen0 if proc/xen and dev/xen are not working although it ...

...is a xen

I had problems on some xen virtual hosts, that /dev/xen/* or /proc/xen was not present. But /dev/xvda1 (partition) was always present.
Is `/dev/xvda1` present for unprivileged domains too? If not, can we merge this check with the line returning `xen0` two lines above?