Bisecting python, I found out 019f0a0cb85ebc234356415f3638b9bd77528e55 ("bpo-34536: raise error for invalid _missing_ results (GH-9147)") causes the issue.
Yet another reduction was possible, and now I can reproduce that python issue on python3.6 as well. I know this is not the best place to report it, but as it's the root cause of this issue and I don't want to waste the investigation effort before I head to a long holiday, here is the reproducer. By the way, I reverted said patch and the other reproducer worked fine, yet have to test with autopkgtest-virt-qemu itself. Cascardo.
import socket def test_tty(): s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) try: raise ValueError("value error") except ValueError as e: exc = e pass test_tty()