Mark packages with missing package.xml unstable

This was discussed in https://github.com/ros/catkin/issues/622

Since I don't have access to the debian buildfarm, someone will need to test this.

Feedback is appreciated.

Thanks,

--scott
I think this needs to be:

    dpkg -c $output_dir/*$distro*.deb | ...

in order to only match one file which `dpkg -c` expects.