sysrestore: Iterate over a list of dict keys
    
    In Python 3, dict.keys() returns a key view. It is not safe to delete
    dict keys when iterating over this view.
    
    Convert the keys to list first.
    
    Part of the work for https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>