Comment 1 for bug 1800511

FWIW I don't think https://github.com/openstack/nova/commit/2b52cde565d542c03f004b48ee9c1a6a25f5b7cd really changed how https://github.com/openstack/nova/commit/f02b3800051234ecc14f3117d5987b1a8ef75877 could have broken anything. _update_vif_xml is called from the source host using migrate data from the dest host, but as far as I know that migrate data doesn't have any information about mtu from the dest to determine what to set in the source vif config. Before _update_vif_xml, we would have just sent the source guest xml vif config to the dest and if the dest didn't support mtu it would have failed also.