parisc/power: Fix power soft-off button emulation on qemu

Make sure to start the kthread to check the power button on qemu as
well if the power button address was provided.
This fixes the qemu built-in system_powerdown runtime command.

Fixes: d0c21947

 ("parisc/power: Add power soft-off when running on qemu")
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v6.0+