conf: Make scheduler formatting simpler
    
    Since the great rework of how we store vcpu- and iothread-related
    data, we have overly complex part of code that is trying to format the
    scheduler tuning data in as less lines as possible by grouping
    settings for multiple threads.  That was designed as an input syntax
    sugar for users, but we don't need to also use that when formatting
    the XML.  Switching to simple enumeration makes the code nicer,
    shorter and more welcoming to future changes.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>