runqemu-ifdown: clean up the remaining iptables rules

runqemu-ifdown: clean up the remaining iptables rules

The iptables rules for the tap interface are added by runqemu-ifup
everytime we use runqemu to start a qemu target. But it's not cleaned
up when runqemu exits.

This patch cleans up the remaining iptables rules for the tap interface
in runqemu-ifdown.

[YOCTO #5047]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>