pc: simplify MachineClass::get_hotplug_handler handling

pc: simplify MachineClass::get_hotplug_handler handling

By default MachineClass::get_hotplug_handler is NULL and concrete board
should set it to it's own handler.
Considering there isn't any default handler, drop saving empty
MachineClass::get_hotplug_handler in child class and make PC code
consistent with spapr/s390x boards.

We can bring this back when actual usecase surfaces and do it
consistently across boards that use get_hotplug_handler().

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-id: 1525691524-32265-2-git-send-email-imammedo@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>