spapr: Use default_machine_opts to set use_hotplug_event_source

spapr: Use default_machine_opts to set use_hotplug_event_source

Instead of setting use_hotplug_event_source at instance_init
time, set default_machine_opts on spapr_machine_2_7_class_options()
to implement equivalent behavior.

This will let us eliminate the need for separate instance_init
functions for each spapr machine-type.

Signed-off-by: Eduardo Habkost <[email protected]>
Message-Id: <20181205205827[email protected]>
Acked-by: David Gibson <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>