[libvirt] [PATCHv2 0/3] create new virSetUIDGID function and use it for qemu

This supercedes a patch I sent yesterday titled: [PATCH] Call initgroups for qemu's uid prior to exec https://www.redhat.com/archives/libvir-list/2010-December/msg00846.html The first patch modifies the lowlevel error logging functions so that they preserve the value of errno. This is necessary because a later patch will rely on errno still being set after returning from a function that logs an error. The second creates a new utility function, for the purpose of centralizing/standardizing use of initgroups to cause a forked process run as a different uid to be a member of *all* groups containing that uid as a member. The 3rd patch changes two separate places in the code to use this new utility function, in both cases fixing some already (or eventually to be) encountered bugs.