ts-debian-hvm-install: Allow udev failure in install media

ts-debian-hvm-install: Allow udev failure in install media

Kernel in "debian-12.1.0-amd64-netinst.iso" prevent debian installer
from booting. Early on, it does `udevadm trigger --action=add`, which
fails, the same way as the following runes fails:

$ cat /sys/devices/virtual/input/input2/name
Xen Virtual Keyboard
$ echo add > /sys/devices/virtual/input/input2/uevent
[   25.884403] synth uevent: /devices/virtual/input/input2: failed to send uevent
[   25.916498] input input2: uevent: failed to send synthetic uevent: -12
sh: write error: Cannot allocate memory
$ uname -a
Linux (none) 6.1.0-10-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.38-1 (2023-07-14) x86_64 GNU/Linux

This doesn't looks like a new issue, Debian Buster ISO seems to do the
same thing (early boot command, and error in Linux logs), so it's
probable that now `udevadm trigger --action=add` return an error when
there's a failure.

Bug report in the kernel and in Debian:
    https://bugzilla.kernel.org/show_bug.cgi?id=207695
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983357

One way to workaround the issue is to remove the vkb device, with xl's
"vkb_device=0", but libvirt doesn't have support for this config
option.

The second option is to edit the installation media, and allow the
failure.

Once installed, the guest boot fine.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>