Cant find file shared with `copy_from_vm` in nixos tests


I am writing a nixos test for a module and incidently the files generated by the test triggers a bug in wireshark.
I would like to export the file to the host to be able to analyze it more comfortably.
I’ve noticed the copy_from_vm feature in the python test driver and tried it in my test but I was not able to find the exported file on the host.
I’ve thus tried with
nix-build -A nixosTests.systemd-analyze which makes use of the feature

I’ve then searched for the result fd -Luuu systemd-analyze.svg /tmp to no avail.
the sharing feature relies on virtfs (look at qemu-vm.nix -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \) so I wonder if this could come from my kernel config, a standard one. lsmod|grep 9p or lsmod|grep virt are empty, could that be it ?