if_vtnet: Support VIRTIO_NET_F_SPEED_DUPLEX

This features lets the guest driver know the speed and duplex of
the "link". Instead of trying to support many media types based
on the possible/likely speeds/duplexes, only use the speed to
set the interface baudrate.

Cleanup ifmedia code to match other drivers.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27908