CNVi is not supported by ASRock Fatal1ty Z370, you will need to get Intel 9260 model.
Here is what ASRock answered me on similar question:
Reference: http://forum.asrock.com/forum_posts.asp?TID=9665&PID=56154�
I also saw that upcoming Z390 boards will support CNVi: https://www.anandtech.com/show/12750/intel-releases-z390-chipset-product-information