power: don't show devices from other seats

Devices that can be assigned to a seat has tag - seat and such
devices are always assigned to one seat. If device is not assigned
to other seat it is implicitly assigned to seat0.

https://www.freedesktop.org/software/systemd/man/sd-login.html
https://www.freedesktop.org/wiki/Software/systemd/multiseat/
7 jobs for wip/muktupavels/multi-seat in 4 minutes and 3 seconds (queued for 46 seconds)
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