qdev: expose DeviceState.hotplugged field as a property

so that management could detect via QOM interface if device was
hotplugged

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
