vhost-user-test qos-test fails on s390x host
Description of problem:
The qos-test is now definitely failing in the ubuntu-20.04-s390x-all CI job. See https://gitlab.com/qemu-project/qemu/-/jobs/3363173491 , then click on "Complete Raw" to see the full log. Quoting:

```
ERROR:../tests/qtest/vhost-user-test.c:248:wait_for_fds: assertion failed: (s->fds_num)
**
ERROR:../tests/qtest/qos-test.c:191:subprocess_run_one_test: child process (/arm/virt/virtio-mmio/virtio-bus/vhost-user-gpio-device/vhost-user-gpio/vhost-user-gpio-tests/read-guest-mem/memfile/subprocess [274051]) failed unexpectedly

(test program exited with status code -6)
```