I was able to get this working, thanks to the corrections provided by @dcsapak and @MoxProxxer. Using your example, it would be:
qm set 104 -args "-fsdev local,security_model=passthrough,id=fsdev0,path=/tmp/share -device virtio-9p-pci,id=fs0,fsdev=fsdev0,mount_tag=hostshare"
This produces the...