Avoid synchronous DBus calls for devices list

Doing a DBus call each time a Device instance is created is costly,
and unnecessary. It is sufficient to query the devices once, and track
added and removed devices using the respective DBus signals.

This also fixes a race when a device is added and removed in a short
time frame. As the frontend calls the backend `createDevice` method,
asking UPower again from the backend may invalidate the just added
device.