Tweak get_virtinstall_images to not include dupes
    
    Some of the version config directives make it possible to wind
    up with dupes - for instance, our config for the 'minimal' image
    can result in multiple instances of VirtInstallImage for F31
    appearing in the list, meaning we'll build the same image two
    or more times in the same run. That's silly! Let's not do that.
    Using a dict keyed on the release number and arch we wind up with
    after interpreting the config file should avoid it.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>