[libvirt] Per-guest configurable user/group for QEMU processes

Hi, This set of patches updates the libvirt's security driver mechanism to support per-guest configurable user and group for QEMU processes running together with other security drivers, such as SELinux and AppArmor. This patches implement the changes discussed in the following thread: https://www.redhat.com/archives/libvir-list/2012-February/msg00990.html Regards, Marcelo Cerri