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.