I downgraded python3-curtin package version 18.1-5-g572ae5d6-0ubuntu1 and confirmed virtio disk type worked fine. It seems the latest, version 20.1-2-g42a9667f-0ubuntu1 causes the issue.
I downgraded python3-curtin package version 18.1-5- g572ae5d6- 0ubuntu1 and confirmed virtio disk type worked fine. g42a9667f- 0ubuntu1 causes the issue.
It seems the latest, version 20.1-2-