In case the 'nbdURI' schema is not known the code would report an error
but wouldn't return failure.
Fixes: 49186372dbe Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Andrea Bolognani <abologna@redhat.com> Tested-by: Andrea Bolognani <abologna@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>