Yes, I can see how this (disabling virtio features) would prevent live
migration from old to new host kernels. You will probably need a
one-time reboot of the guest when migrating to the new host kernel.
We could either mention this in the DSA text, or keep UFO enabled even
though it doesn't work correctly (in practice, we sort of have to do
that as the tap device's feature flags aren't respected by guests -
which I think is a QEMU bug).
Please let us know whether live migration between two hosts running the
new kernel version does work (I think it will).