After r358180, all network backends (tap, netmap) support MTU > 1500. So, I think it would be useful to be able to give a hint to the guest system about host MTU.
The motivation for this change can be seen here: https://lists.oasis-open.org/archives/virtio-dev/201601/msg00002.html
Specification: https://docs.oasis-open.org/virtio/virtio/v1.1/csprd01/virtio-v1.1-csprd01.html (5.1.3 Feature bits)
Please note, that this feature is optional. Some OS's are too old. Some, such as FreeBSD, do not support this.
I want to add support for the FreeBSD if_vtnet.c driver in a separate review.