Oh, OK.You need to make a loader tunable, not edit the file directly if you want it to continue to work after a reboot.
TrueNAS has been rebooted since and if I run kldstats I can see virtio_console.ko in the list.
I'm afraid the term "tunable" is new to me. When you say make a loader tunable, is just a file with the contents as you have printed above? What is the file extension and where would I need to place this file? Also, would I need to remove the line I added to loader.conf to load to the virtio_console module?