conf: introduce 'autodeflate' attribute for memballoon device
    
    Excessive memory balloon inflation can cause invocation of OOM-killer,
    when Linux is under severe memory pressure. QEMU memballoon device
    has a feature to release some memory at the last moment before some
    process will be get killed by OOM-killer.
    
    Introduce a new optional balloon device attribute 'autodeflate' to
    enable or disable this feature.