qdev: add helpers to be more explicit when using abstract QOM parent functions

QOM API learning curve is quite hard, in particular when devices inherit from
abstract parent.
To be more explicit about when a device class change the parent hooks, add few
helpers hoping a device class_init() will be easier to understand.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180114020412.26160-3-f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>