Comment 6 for bug 1776269

Actually checking the issue in more detail it seems even more that neither libvirt nor qemu is "it".

Instead many users report similar issues [1][2] due to the host grabbing (part of) the device.
That leads to a conflict then.

So I'd ask to check if any of that applies to your bionic system, maybe it has a newer or additional driver that does this than what your 17.10 had.

[1]: https://www.redhat.com/archives/vfio-users/2016-March/msg00088.html
[2]: https://www.reddit.com/r/VFIO/comments/4sxt3h/cant_get_pci_passthrough_quite_right/