Starting libvirtd.service provides iptable rules for NATed network virbr0.
When starting firewalld.service after libvirtd, these rules will be
overwritten. So NAT will no longer work. Restart of libvirtd fixes the
rules again.
When terminating screen lock, pam_exec is called in the context of the corresponding user.
Non-root users don't have the permission to start/stop firewalld. So exit immediately.