indexerror in vmbuilder when partitionfile contains 2 columns or less

Binary package hint: python-vm-builder

==================================
SRU justification:
1. Impact: using a partition file which doesn't container *optional* elements causes failure.
2. How bug addressed: the code is patched to use a proper test for the presence of the optional elements. (Existing check was simply wrong)
3. Patch: See comment number 4
4. To reproduce:
cat > partfile << EOF
root 4000
swap 1000
EOF
vmbuilder xen ubuntu --part partfile
5. Regression potential: If the patch were wrong, other bugs in the parsing of the partfile could be introduced.
==================================

In 11.04 with python-vm-builder 0.12.4+bzr464-0ubuntu1, when you have a file vmbuilder.partition with 2 lines:
root 4000
swap 1000

and you run "vmbuilder xen ubuntu --suite natty --flavour virtual --arch amd64 -o --ip 192.168.122.2 --hostname myvm --bridge virbr0 --part vmbuilder.partition --user user --name user --pass default" it will crash with this error:
Traceback (most recent call last):
  File "/usr/bin/vmbuilder", line 24, in <module>
    cli.main()
  File "/usr/lib/python2.7/dist-packages/VMBuilder/contrib/cli.py", line 222, in main
    self.set_disk_layout(hypervisor)
  File "/usr/lib/python2.7/dist-packages/VMBuilder/contrib/cli.py", line 351, in set_disk_layout
    if not elements[3]:
IndexError: list index out of range

This is fixed with the patch i included