Add VIRTIO_NET_F_MTU flag support for the bhyve virtio-net device.
The flag can be enabled using the new 'mtu' option:
bhyve -s X:Y:Z,virtio-net,[tapN|valeX:N],mtu=9000

Reported by: vmaffione, jhb
Approved by: vmaffione (mentor)
Differential Revision: https://reviews.freebsd.org/D23971