The current user might not be able to peek into the loop files directly,
use the helper. Discovered by running tests in travis.
Signed-off-by: David Sterba <dsterba@suse.com>
Create filesystem on a partitioned loop device, test for "btrfs-progs:
Fix partitioned loop devices resolving".
Signed-off-by: David Sterba <dsterba@suse.com>