Oups, edit to the previous mail.
>     $ sudo systemctl enable acpid
>     $ sudo systemctl status acpid
In the second line, replace status with start.
Moreove, `acpid` requires additional configuration that I forgot to
mention. It seems that by default it does nothing.
So, let's create an event to handle the power button.
    $ echo 'event=button/power
    action=/etc/acpi/actions/poweroff.sh %e
    ' > /etc/acpi/events/power-button
Then, create the action script to handle this event.
    $ mkdir /etc/acpi/actions
    $ echo '#!/bin/sh
    /sbin/shutdown -h now
    ' > /etc/acpi/actions/poweroff.sh
    $ chmod +x /etc/acpi/actions/poweroff.sh
With that, it seems now that the VM shuts down properly on `virsh shutdown`.
Cheers,
Arnaud