if_vtnet: Defer updating generated MAC address until attached

This improves spec compliance because the driver is not suppose
to notify the device prior to setting the DRIVER_OK status, which
could happen with the VIRTIO_NET_F_CTRL_MAC_ADDR.

The VIRTIO_NET_F_MAC feature should always be negotiated so would
be a rare situation.

Reviewed by: grehan (mentor)
Differential Revision: https://reviews.freebsd.org/D27910