In qemu arm64 with "virt" machine, with ACPI firmware,
```
# hexdump /dev/input/event2
```
and in qemu monitor
```
(qemu) system_poweroff
```
and make sure some power button input event appear.
( setting sysctl hw.acpi.power_button_state=S5 is not work,
because ACPI tree does not have \_S5 object.)