storage: Add writelabel bool for virStorageBackendDeviceProbe
    
    It's possible that the API could be called from a startup path in
    order to check whether the label on the device matches what our
    format is. In order to handle that condition, add a 'writelabel'
    boolean to the API in order to indicate whether a write or just
    read is about to happen.
    
    This alters two "error" conditions that would care about knowing.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>