util: fix virStorageFileGetBackingStoreStr error handling
    
    The virStorageFileGetBackingStoreStr method has overloaded the NULL
    return value to indicate both no backing available and a fatal
    error dealing with it.
    
    The caller is thus not able to correctly propagate the error
    messages.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>