BackingFile property should be use with StorageVolume instead of StorageAccess
BUG: 434617 When mounting an iso with `udisksctl`, first a loop device is created `/dev/loop0` then a second device `/dev/loop0p1` is used to mount it on a filesystem location. Querying for StorageAccess devices sometimes only returns the `/dev/loop0p1` device without the `/dev/loop0` device and the BackingFile property does not work on `/dev/loop0p1`. Solution: query for StorageVolume instead which returns all loop devices: `/dev/loop0` and `/dev/loop1`. Warning: Because StorageVolume returns more devices, the function `getDeviceFromBackingFile` could be a little bit slower.