Linux power_supply sysfs interface allows checking if the battery powers
the whole system or a specific device/tree of devices with `scope`
attribute[1]. We can use it to skip the non-system power supplies in the
battery module and avoid adding HIDs or other peripheral devices to the
total.
The logic is based on UPower, where it is assumed that "Unknown" devices
or devices without a `scope` are system power supplies.
[1]: https://lore.kernel.org/lkml/alpine.LNX.2.00.1201031556460.24984@pobox.suse.cz/T/
22084691ff