It appears that input/input.c is responsible for the insane modalias
length.  If I am reading input_print_modalias() correctly, it appends a
"k" plus every key code that the keyboard supports, and the Xen Virtual
Keyboard supports a lot of keycodes.  Why does it do this?

Phillip Susi writes:

So I have finally drilled down to the modalias for the Xen Virtual
Keyboard driver being so long ( over 2KB ) that it causes an -ENOMEM
when trying to add it to the environment for uevents.  This causes
coldplug to fail, which causes the script doing coldplug as part of the
debian-installer init to fail, which causes a kernel panic when init
exits, which then for reasons I have yet to understand, causes the Xen
domU to reboot.

Why is this modalias so huge?  Can we pare it down, or or is there
another solution to get uevents working on this device again?  Maybe the
environment block size needs to be increased?  I don't know.