conf: Add callbacks that allocate per-def private data
    
    Some drivers use def-specific private data across callbacks (e.g.
    qemuCaps in the qemu driver). Currently it's mostly allocated in every
    single callback. This is rather wasteful, given that every single call
    to the device callback allocates it.
    
    The new callback will allocate the data (if not provided externally) and
    then use it for the VM, address and device post parse callbacks.