We believe that your issue has been closed by the upload of
Version 4.0~a13-1+grml.3 of live-boot from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Hash: SHA1

Format: 1.8
Date: Sat, 28 Sep 2013 11:44:52 +0200
Source: live-boot
Binary: live-boot-grml live-boot-grml-doc live-boot-grml-initramfs-tools
Architecture: source all
Version: 4.0~a13-1+grml.3
Distribution: unstable
Urgency: low
Maintainer: Grml Team <team@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description: 
 live-boot-grml - Live System Boot Scripts
 live-boot-grml-doc - Live System Boot Scripts (documentation)
 live-boot-grml-initramfs-tools - Live System Boot Scripts (initramfs-tools backend)
Changes: 
 live-boot (4.0~a13-1+grml.3) unstable; urgency=low
 .
   * [79dceec] Update 9990-misc-helpers.sh to support latest udev version
     [Closes: issue1278] Thanks to Michael Biebl <biebl@debian.org> for
     helping in resolve this issue and Ulrich Dangel <mru@spamt.net>
     for the initial patch
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 99dfbc4557e2c015cc7971e6855dd724d8d6e4ed 37060 live-boot_4.0~a13-1+grml.3.debian.tar.xz
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FTR, the 'udevadm test-builtin' call in question comes from /lib/live/boot/9990-
misc-helpers.sh (see http://paste.grml.org/1529/).

With hwdb.bin included in the initramfs (still fails):

  http://screeni.s3.amazonaws.com/screenshot.2013-09-27T23:59:36.png

Trying to boot recent daily ISOs which were considered as base for stable release 2013.09 
failed to boot via "kvm [...] -hda $ISO":

  http://screeni.s3.amazonaws.com/screenshot.2013-09-27T22:55:34.png

I'm afraid this is a problem of libudev0 v175-7.2 (as present in Grml 2013.09-rc1) vs 
libudev1 v204-5 (as present in current dailys). Might be enough to just ship this file 
via the initramfs, didn't investigate/test that though.