tests: Allow testing large files
    
    util.create_file() was filling the file with "x". This works for small
    files but prevent testing big files. Change to create sparse file so we
    can test easily large files and offsets.
    
    Tests that want to fill areas on storage can access the file directly.
    We don't have such tests yet.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>