linux: Only one warning if no valid voltage found

Avoid filling the logs with repeated warning saying that no correct
voltage value was found for a power device. Improves the situation with
bugs https://bugzilla.redhat.com/show_bug.cgi?id=847874 and
https://bugzilla.redhat.com/show_bug.cgi?id=863524

Signed-off-by: Timothée Ravier <tim@siosm.fr>
Signed-off-by: Martin Pitt <martinpitt@gnome.org>
linux: Detect the battery of bluetooth input devices.

First, checking the device path. If it contains 'bluetooth', it is a bluetooth
devices. Second, checking if there is an input folder. If there is mouse
folder under the input folder, it is a mouse device. Otherwise, it is a
keyboard device.