Qemu scripts are not disabled when using "script=no"

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SRU Justification:
1. Impact: default qemu-ifdown script is bad
2. Development fix: remove hardcoded bad paths in /etc/qemu-ifdown
3. Stable fix: same as development fix
4. Test case: run qemu (as root) as shown in description:
     a. qemu-img create x.img 1G
     b. kvm -vnc :1 -hda x.img -net tap,ifname=tap0,script=no
     c. kill it with ctrl-c
   At shutdown, it will fail due to a bad path in the tap down script.
5. Regression potential: if users are running kvm with a bad path they might not find the executables. However as brctl's path is bad (has been changed) it can't really regress.
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I used kvm with args which looks like this:
kvm -hda disk.img -net nic,macaddr=0:1:2:a:a:1 -net tap,ifname=tap0,script=no

I created myself a tap0, enslaved in my own bridge br0.
After a shutdown, I got this message (which corresponds to the bug #833475):
/etc/qemu-ifdown: 8: /usr/sbin/brctl: not found

The issue in my case, is the fact I use "script=no" to avoid these scripts, in order to use my own scripts. I didn't try, but I can suppose "downscript=no" is also impacted.

Regards

ProblemType: Bug
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
BootDmesg:
CurrentDmesg:
Date: Wed Nov 30 17:12:21 2011
Dependencies:
DistroRelease: Ubuntu 11.10
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
KvmCmdLine:
Lspci:
Lsusb:
MachineType: Dell Inc. Precision WorkStation T3400
NonfreeKernelModules: nvidia
Package: qemu-kvm 0.14.1+noroms-0ubuntu6
PackageArchitecture: amd64
ProcCpuinfo:
ProcEnviron:
ProcInterrupts:
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.0.0-12-generic root=UUID=fa2e6fc4-5251-4da0-86cd-1e3e0559181b ro quiet splash vt.handoff=7
ProcModules:
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
RelatedPackageVersions:
SourcePackage: qemu-kvm
Tags: oneiric
UdevDb:
UdevLog:
Uname: Linux 3.0.0-12-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 08/14/2008
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A08
dmi.board.name: 0TP412
dmi.board.vendor: Dell Inc.
dmi.chassis.asset.tag: B015127
dmi.chassis.type: 7
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA08:bd08/14/2008:svnDellInc.:pnPrecisionWorkStationT3400:pvr:rvnDellInc.:rn0TP412:rvr:cvnDellInc.:ct7:cvr:
dmi.product.name: Precision WorkStation T3400
dmi.sys.vendor: Dell Inc.