[UDisks2 Backend] Don't do media check for loop devices
They cannot really have a physical drive attached to themselves
Saves a significant amount of time on a setup with lots of snaps.
Brings udisks2 enumeration time on Dolphin startup on my machine with roughly 30 loop mounts down from ~160ms to ~20ms.
My mounted CD ISO images don't have a drive, either.
Edited by Kai Uwe Broulik