kvmtool: Introduce downscript option for virtio-net

To detach tap device automatically from bridge when exiting,
just like what the reverse of "script" does.

Signed-off-by: Fan Du <fan.du@intel.com>

include/kvm/virtio-net.h

@@ -9,6 +9,7 @@ struct virtio_net_params {
 	const char *guest_ip;
 	const char *host_ip;
 	const char *script;
+	const char *downscript;
 	const char *trans;
 	const char *tapif;
 	char guest_mac[6];