Fix failure to start old containers

The existence of $root/var/lib/private/host-notify as a socket
prevented a bind mount:

  container foo[8083]: Failed to create mount point /var/lib/containers/foo/var/lib/private/host-notify: No such device or address

nixos/modules/virtualisation/containers.nix

@@ -338,7 +338,7 @@ in
             fi
           ''}
 
-
+          rm -f $root/var/lib/private/host-notify
 
           # Run systemd-nspawn without startup notification (we'll
           # wait for the container systemd to signal readiness).