param-validation: add static creation methods to bt_param_validation_value_descr

param-validation: add static creation methods to bt_param_validation_value_descr

Replace the various constructors of bt_param_validation_value_descr with
equivalent static methods.

Change-Id: Ia53e05369df9f5f357f1e9eed72d7cfc8c6cb11b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7209
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>