Comment 24 for bug 1664737

That is the new libvirt 4.0 which gives the better messages.
Interesting.

Could be an arm specific lack of virtio (which is generic).
IIRC there was something that arm only worked with modern (virtio 1) and might force this.
But I could be wrong (too old info).

The XML as well as the qemu command you linked had no version set.
In XML that would be like:
<virtio revision='1.0'/> (or 0.9 )
And on the commandline I'd expect either of the controls you already found disable-modern/disable-legacy.

You could try if openstack can leave the guest/volumes around.
Then you could login the host and tweak the xml for virtio revision.
With that it should generate modern/legacy accordingly.
And that might make it work.

Once confirmed if that "is it" we can still think how to make that working in openstack.
Can you test that locally?