This guide will fix the issue of making Windows VPS on QCOW2 storage on CentOS 7.3.
Issue was occurring because on CentOS 7.3 they have removed the support of guestmount and due to that QCOW2 images were not able to mount and hence the issue.
Execute the following command to fix it :
root# /usr/local/emps/bin/php /usr/local/virtualizor/scripts/guestfs_fix.php
After this try to create VPS on CentOS 7.3 and QCOW2 storage, you should not face any error.