Thanks for the reply @leesteken !
I have added
args: -drive if=none,id=stick,format=raw,file=/var/lib/vz/template/iso/pfconfig-test.img -device nec-usb-xhci,id=xhci -device usb-storage,bus=xhci.0,drive=stick
as documented in
https://qemu-project.gitlab.io/qemu/system/devices/usb.html
The VM...