vm: allow user-define hook for IP change event

This is especially useful for proxy VMs that e.g. run some transparent proxy service such as tor,
and need to rebind it upon IP change (of course this assumes iptables-based transparent redirection
such as DNAT).

common/setup_ip

@@ -19,6 +19,7 @@ if [ x$ip != x ]; then
 		echo "NS1=$gateway" > /var/run/qubes/qubes_ns
 		echo "NS2=$secondary_dns" >> /var/run/qubes/qubes_ns
 		/usr/lib/qubes/qubes_setup_dnat_to_ns
+		[ -x /rw/config/qubes_ip_change_hook ] && /rw/config/qubes_ip_change_hook
 	fi
 
 fi