Filesystems output from running "vmbuilder xen ubuntu..." are named confusingly

When creating a xen based ubuntu VM using vmbuilder, the filesystems copied to the ubuntu-xen folder are named in a confusing and not very helpful way, namely, "tmp" followed by a random string. This isn't too helpful if you're trying to use vmbuilder programmatically, e.g. from a script.

I've attached a simple patch that allows you to specify a fourth option in a part file added through the --part switch which specifies a name of the output file.

E.G.
Using this part file:

$ cat part
root 1024 a1 root
/mnt 1 b mnt

will result in the following output:

$ ls -Alh ubuntu-xen
total 498M
-rw-r--r-- 1 keyz keyz 1.0M 2010-12-09 19:42 mnt
-rw-r--r-- 1 keyz keyz 1.0G 2010-12-09 19:42 root
-rw-r--r-- 1 keyz keyz 420 2010-12-09 19:42 xen.conf

and from xen.conf:

$ cat ubuntu-xen/xen.conf
...
disk = [
'tap:aio:/home/keyz/vm/ubuntu-xen/root,xvda1,w',
'tap:aio:/home/keyz/vm/ubuntu-xen/mnt,xvda2,w'
]
...

I'm not sure if this is the best place for this, if not, apologies, can someone point me where to go.