libvirt: Move config getters to util

libvirt: Move config getters to util

virConnectGetConfig and virConnectGetConfigPath were static libvirt
methods, merely because there hasn't been any need for having them
internally exported yet. Since libvirt-admin also needs to reference
its config file, 'xGetConfig' should be exported.
Besides moving, this patch also renames the methods accordingly,
as they are libvirt config specific.