custom dd-tgz format images looked for in wrong path, so they don't work

I applied the patch for bug https://bugs.launchpad.net/maas/+bug/1393953 to my existing MAAS installation which is at this version:

Installed: 1.7.0~rc3+bzr3299-0ubuntu1~trusty1

(to note, the patch in question is now in MAAS trunk at revno 3408)

After doing this, dd-tgz format images don't work. I uploaded the image like so:

maas roadmr boot-resources create name=kitty-t1 title='kitty test 1' architecture=amd64/generic content@=/home/roadmr/download/kitty-dd.tgz filetype=ddtgz

but when trying to deploy it to a node it seems to still be looking for the root-tgz (rather than root-dd) file:

--2014-12-10 19:42:05-- http://10.10.10.1/MAAS/static/images/custom/amd64/generic/kitty-t1/uploaded/root-tgz
Connecting to 10.10.10.1:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2014-12-10 19:42:05 ERROR 404: Not Found.

I traced this to the fact that the method is missing a path component when it builds the path where it should look for the dd file. I see it looking for e.g.
/var/lib/maas/boot-resources/custom/amd64/generic/kitty-t1/uploaded/root-dd but the file is actually in
/var/lib/maas/boot-resources/current/custom/amd64/generic/kitty-t1/uploaded/root-dd (note extra "current" component).

I think simply adding the missing component to provisioningserver/drivers/osystem/custom.py in the get_xinstall_parameters method (the path variable definition) would fix things.