qdev: Set device's parent before calling realize() down inheritance chain

Currently device_set_realized() sets parent only after device was realized,
but qdev_device_add() sets it before device is realized.
Make behavior consistent and alter device_set_realized() to behave like
qdev_device_add().

It will allow to set link<> properties in realize() method in classes
inherited from DEVICE.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>