valFn = function (v) {
            if (v == null) return true;
            return __isGuid(v);
        }
Returns a Guid data type Validator.
    @example
        // Assume em1 is a preexisting EntityManager.
        var custType = em1.metadataStore.getEntityType("Customer");
        var customerIdProperty - custType.getProperty("CustomerID");
        // Validates that the value of the CustomerID property on Customer is a Guid.
        customerIdProperty.validators.push(Validator.guid());
    @method guid
    @static
    @return {Validator} A new Validator