virsh migrate --live GuestName DestinationURL
This is feasible when network storage is available and useful when a host needs to be drained for maintenance.My team spent 4 months on our implementation of vm memory that let us do it and its still our biggest time suck. We also were able to make assumptions like RDMA that are not available.
All that to say — as someone not working on smolVMs — I am confident smolVMs and most other OSS sandbox implementations will get live migration via hypervisor upgrades in the next 12 months.
Until then there are enterprise-y providers like that have it and great OSS options that already solve this like cloud hypervisor.