Figured it out. Using the AHCI/Intel E1000 configuration should work. But I couldn't get it to work reliably. This
bhyve/Windows procedure recommends creating your own Windows ISO spin and using an auto installation script. If I had dozens of Windows VM to create, that might be a better way. Since I only needed one, I streamlined the procedure...
1. Put Windows ISO on FreeNAS share.
2. Put
VirtIO driver ISO on FreeNAS share.
3. Create VM with TWO CD-ROMs using the above ISOs.
4. Boot from the Windows ISO, grab the drivers off the other ISO.
5. Install Windows.
6. Be happy.
Other tips...
If you want your VM to start properly on reboot of FreeNAS without any human intervention, make sure you do check 'Autostart' and do NOT check 'Wait to boot:' in the VNC device configuration.
Cheers,
Matt