nixos/libvirtd: don't use cp -n

nixos/modules/virtualisation/libvirtd.nix

···
             libvirt/nwfilter/*.xml );
         do
             mkdir -p /var/lib/$(dirname $i) -m 755
-            cp -npd ${cfg.package}/var/lib/$i /var/lib/$i
+            if [ ! -e /var/lib/$i ]; then
+              cp -pd ${cfg.package}/var/lib/$i /var/lib/$i
+            fi
         done
 
         # Copy generated qemu config to libvirt directory