Add a helper that will check if the loop devices is backed by
the file identified via the stat(2) result, i.e. the inode on
the correspoding device.
Add a correspoding test for the new helper.
Implement a `Loop.get_status` method, to get the properties of the
loop device, corresponding to LOOP_GET_STATUS64, and counterpart
to the existing `Loop.set_status` method. Use the new `get_status`
call in the `set_status` call, replacing the existing code that
does the same thing.
Add a basic test for the `get_status` method. Also fix an actual
leak, where the loop device was closed but the fd was not cleared
inside the test.