python-lxc can't create container (bad address)

[Chris West]

Container..create(.. fails when the equivalent lxc-create command succeeds fine:

% python -c 'import lxc; print(lxc.Container("new").create("download", 0, {"dist": "ubuntu", "release": "utopic", "arch": "amd64"}))'
lxc_container: lxccontainer.c: create_run_template: 1116 Bad address - failed to execute template lxc-usernsexec
lxc_container: lxccontainer.c: create_run_template: 1121 container creation template for new failed
False

% lxc-create -n new -t download -- -d ubuntu -r utopic -a amd64
Using image from local cache
Unpacking the rootfs

---
You just created an Ubuntu container (release=utopic, arch=amd64, variant=default)
The default username/password is: ubuntu / ubuntu
To gain root privileges, please use sudo.

This is for limited user lxc, with .config/lxc/default.conf:
lxc.id_map = u 0 100000 65536
lxc.id_map = g 0 100000 65536
lxc.network.type = veth
lxc.network.link = lxcbr0

And /etc/lxc/lxc-usernet:
faux veth lxcbr0 20

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: python-lxc 0.1-0ubuntu2
ProcVersionSignature: Ubuntu 3.16.0-21.28-generic 3.16.4
Uname: Linux 3.16.0-21-generic x86_64
ApportVersion: 2.14.7-0ubuntu5
Architecture: amd64
Date: Fri Oct 10 17:05:42 2014
InstallationDate: Installed on 2014-08-01 (70 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: python-lxc
UpgradeStatus: Upgraded to utopic on 2014-09-11 (29 days ago)