This branch uses tc to fixup the checksum. It's appearing to work with the few guests I've tested so far, but there is an issue when a network has <bandwidth> configured, and I don't want to post the patches for review until I figure that out. In the meantime anyone who wants to test with a non-<bandwidth> network, please do :-)
https://gitlab.com/lainestump/libvirt/-/tree/network-freebsd-checksum-fix-tc2
