Better to fix than to guess if there are users, I think.
Yes, but it will require some knowledge of s390x, which I cannot provide.
Commit 57da367b9ec4 ("s390x/pci: forbid multifunction pci device") says having
a multifunction device will make the guest spin forever. That is not what Cédric observed
with igb so it may no longer be relevant, but I cannot be sure that the problem is
resolved without understanding how multifunction devices are supposed to work with s390x.
Ideally someone with s390x expertise should check relevant hardware documentation and confirm QEMU properly implements mutlifunction devices.