vmbuilder kvm doesn't create guest domain

I'm trying to setup a KVM Ubuntu guest. I use vmbuilder. When vmbuilder cli returns I see the image files in ubuntu-kvm/ but virsh list --all doesn't list any new domain and I can't start the guest.

This is the command I issue:

# vmbuilder kvm ubuntu --suite precise --flavour virtual --arch i386 -o --ip 192.168.42.180 --hostname davide --part vmbuilder.partition --user davide --name davide --pass davide -vv 2> err_log

These are the contents of the vmbuilder.partition file:

root 20000
swap 4000
---

This is the output of vmbuilder (err_log file):
http://paste.ubuntu.com/1609761/

Other info:

The host is running Ubuntu 12.04.1 LTS Server

uname -a:
Linux tornado.thmdev.com 3.2.0-33-generic #52-Ubuntu SMP Thu Oct 18 16:29:15 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

virsh version:
Compiled against library: libvir 0.9.8
Using library: libvir 0.9.8
Using API: QEMU 0.9.8
Running hypervisor: QEMU 1.0.0

Version of python-vm-builder package:
0.12.4+bzr477-0ubuntu1