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

This (partially) reverts commit 24c160cee85d781d74c0018936936091a64337ad.

See #966.

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.

People who have a chroot running with the current HEAD, on dev channel, must exit the chroot before updating (`enter-chroot` would fail: I do not want to add an extra test for such a short-lived commit).

Sorry about that one, I should have tested it (we miss you autotesters....)...