Provide workaround for Debian bug #918590 with lvm + udev in VM installations

We need the /run/udev bind-mount to be present also when doing VM
installations for buster and newer, otherwise the installation
takes loooong due to the:

| WARNING: Device /dev/... not initialized in udev database even after waiting 10000000 microseconds

bug. As the /run/udev bind-mount is already gone we need
to resurrect before invoking update-grub in the VM installation.