The discussion upstream does not seem to be converging on a proper fix in the kernel, so I'm going to clone this bug and suggest that debian-installer patch the start-udev script to ignore the failure of the udevadm trigger command.
To summarize: init ends up calling start-udev which calls udevadm trigger to cold plug all devices. Both scripts are set -e. The Xen Virtual Keyboard driver and at least one other driver have always failed to trigger due to having absurdly long modalias, but the error used to be ignored. The kernel now returns the error to udevadm, so it exits with an error, so start-udev exits with an error, so init exits with an error, causing the kernel to panic.
