[libvirt] Dropping RHEL-6/CentOS-6 support

Dear list, I've noticed a failed build on CentOS-6 after some commits. Problem was that old gcc is not wise enough and produces a false positive. I've proposed a patch for that [1] but honestly, neither am I - like Peter - very fond of this approach. We should not try to fix a good code because of some spurious warnings. Moreover if they happen on a system that is considered stable and thus nobody should run recent libvirt on it. In RHEL-6/CentOS-6 there's libvirt-0.10.2 which is 3.5 years old now. I'm starting this thread so that the decision and discussion is clear and not buried under discussion to the patch. If we happen to stop caring we probably should stop our CentOS-6 build in jenkins too [2]. Michal 1: https://www.redhat.com/archives/libvir-list/2016-February/msg00407.html 2: https://ci.centos.org/view/libvirt-project/job/libvirt-daemon-build/syste...