libvfio-user test-client-server test hanging in the CI

[Thomas Huth]

 Hi Elena, Jagannathan & John!

I noticed that the libvfio-user test-client-server test is recently hanging 
in the check-system-centos jobs in QEMU's gitlab-CI. Normally the test 
finishes within 20 minutes, see e.g. here:

 https://gitlab.com/qemu-project/qemu/-/jobs/2928561403

(notice that test-client-server has number 252/274 here).

But in many recent runs, it seems to hang after number 251:

 https://gitlab.com/qemu-project/qemu/-/jobs/2922743324
 https://gitlab.com/thuth/qemu/-/jobs/2934299005
 ...

Could you please have a look? Also, why is the test not timing out after 
some minutes? ... completely blocking the CI job for the rest of the hour 
'til the global 60 minutes timeout kicks in is not very nice :-(

 Thanks,
  Thomas