Plan9 file-system works nicely with KVM. We use it in our linux (Devuan) based hypervisors for data volumes...
Strong points are the permission mapping (possible per-VM on same share) and ease of use. It's greqt to back the volume with a ZFS dataset in the hypervisor...
However, 9pfs is less suited to certain workloads. In particular, it has poor database storage compatibility and suffers from very-poor small-file performance (if sync writes are needed).
In KVM, virtiofs is much more performant...