Revert "enter-chroot: Simply bind mount /var/run/udev" (+ invert test)

This reverts commit 24c160cee85d781d74c0018936936091a64337ad.

Bind mounting in /var/run is really a bad idea: we need to
hardcode the path in installer/prepare/ubuntu, or run another
find/xargs expression.

Bind mounting to /var/host is simpler, and works just fine.