Allow koan to install "image" based virtual machines

Currently, koan reproducibly breaks when trying to access a virtual machine definition, if the virtual machine is inheriting an "image":

    $ koan --display --system=system1
    DEBUG: get_data(<koan.app.Koan instance at 0x7f613ea0bd40>,system,system1)
    ...
    Fault: <Fault 1: "<type 'exceptions.AttributeError'>:'NoneType' object has no attribute 'arch'">

And in cobbler.log:

      File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 1970, in _dispatch
        return method_handle(*params)
       File "/usr/lib/python2.6/site-packages/cobbler/remote.py", line 1481, in get_system_for_koan
        arch = distro.arch