bhyve: Handle snapshots of unconfigured virtio-net devices

In case of device reset or not configured - features_negotiated is not
set, calling calling pci_vtnet_neg_features is wrong and resume gets
"Segmentation fault".

Reviewed by: markj
Sponsored by: vStack
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36244

(cherry picked from commit 3b5e5ce87bc885f35100d77b31ac81d3f98342bf)