[packagekit] Fixing some bugs in the conary backend

[Jesse Zhang]

Hi,

My first attempt at the packagekit/conary code, so I'd appreciate any of your
comments.

Patch 1-3 update the conary backend for the internal API change.

Patch 4 fixes a bad behavior that's been there for a long time. When the
repository cache is broken (e.g. user presses Ctrl-C before the XML file
finishes downloading), packagekit will stop working ever after.

Patch 5 adds some unit-tests. But I'm not sure if there is some established
practices with regard to testing.

I see you are planning a release, so I'm not keen about getting this in too
early :)

jesse