Missing destination files

It looks like this bug is back, but I'm opening a new bug for the issue.

In the latest system-image tests, we ask for files to be downloaded in unconstrained mode, but while udm comes back with a successful group download, the si assertion for the local destination files fails. From the si log file:

[systemimage] Jan 21 10:20:37 2015 (18788) [0x7f87aadf3ba8] Running group download reactor
[systemimage] Jan 21 10:20:37 2015 (18788) [0x7f87aadf3ba8] Group download reactor done
[systemimage] Jan 21 10:20:37 2015 (18788) check_for_update failed
Traceback (most recent call last):
  File "/home/barry/projects/si/trunk/systemimage/api.py", line 99, in check_for_update
    self._state.run_until('download_files')
  File "/home/barry/projects/si/trunk/systemimage/state.py", line 178, in run_until
    step()
  File "/home/barry/projects/si/trunk/systemimage/state.py", line 218, in _get_blacklist_1
    get_keyring('blacklist', url, 'image-master')
  File "/home/barry/projects/si/trunk/systemimage/keyring.py", line 107, in get_keyring
    (ascxz_src, ascxz_dst),
  File "/home/barry/projects/si/trunk/systemimage/download.py", line 201, in get_files
    self._get_files(records, pausable)
  File "/home/barry/projects/si/trunk/systemimage/udm.py", line 180, in _get_files
    missing, local_paths))
AssertionError: Missing destination files: ['/tmp/tmp84xda3e3/ubuntu/cache/keyring.tar.xz', '/tmp/tmp84xda3e3/ubuntu/cache/keyring.tar.xz.asc']
local_paths: [dbus.String('/tmp/tmp84xda3e3/android/cache/5.txt'), dbus.String('/tmp/tmp84xda3e3/android/cache/5.txt.asc'), dbus.String('/tmp/tmp84xda3e3/android/cache/6.txt'), dbus.String('/tmp/tmp84xda3e3/android/cache/6.txt.asc'), dbus.String('/tmp/tmp84xda3e3/android/cache/7.txt'), dbus.String('/tmp/tmp84xda3e3/android/cache/7.txt.asc')]

The exact same test using pycurl passes, since the files do end up in the requested destination.