[bhyve] virtio-net: Do not allow receiving packets until features have been negotiated.

Enforce the requirement that the RX callback cannot be called after a reset until the features have been negotiated.
This fixes a race condition where the receive callback is called during a device reset.

Reviewed by: vmaffione, grehan
Approved by: vmaffione (mentor)
Sponsored by: vstack.com
Differential Revision: https://reviews.freebsd.org/D27381