getOldStyleBlockDevice: Adjust formatting
    
    Instead of initializing return value to zero (success) and overwriting
    it on every failure just before the control jumps onto 'out' label,
    let's initialize to an error value and set to zero only when we are
    sure about the success. Just follow the pattern we have in the rest of
    the code.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>