ZFS supports instant resizing of datasets. When that dataset is the virtual disk for a VM you can just increase its size on the hypervisor, then it's a simple growfs operation for the VM to see the increased size. On LXC the dataset is usually mounted directly so the resize operation is reflected immediately.

I use Proxmox as the hypervisor, and the ZFS resize part is supported on the GUI and it's trivial to use. Let me know if you need more details.