/etc/qemu-ifdown is in qemu-system-common 1:2.11+dfsg-1ubuntu7.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | #! /bin/sh
# Script to shut down a network (tap) device for qemu.
# Initially this script is empty, but you can configure,
# for example, accounting info here.
:
|