H5VL_IS_CONNECTOR_REGISTERED_BY_VALUE tests whether a VOL class has been registered or not, according to the supplied connector value
connector_value has a type of H5VL_class_value_t, which is defined in
typedef int H5VL_class_value_t;
Valid VOL connector identifiers can have values from 0 through 255 for connectors defined by the HDF5 library. Values 256 through 511 are available for testing new filtersconnectors. Subsequent values should be obtained by contacting the The HDF Help Desk.