CentOS7: KVM: error: Cannot create user runtime directory '/run/user/0/libvirt': Permission denied

By default non-root users cannot access libvirtd directly, unless explicitly granted authorization.

I've done this using PolicyKit:

# cat /etc/polkit-1/rules.d/50-org.libvirt.unix.manage.rules
polkit.addRule(function(action, subject) {
        if (action.id == "org.libvirt.unix.manage" &&
            subject.user == "nrpe") {
                return polkit.Result.YES;
                polkit.log("action=" + action);
                polkit.log("subject=" + subject);

This will let user nrpe do whatever they want to do with libvirtd without requiring a password.

Second, non-root users need to specify the connection URL explicitly in order to access the system libvirtd.

virsh --connect qemu:///system list --all