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 <ehabkost@redhat.com>
Message-Id: <20181205205827.19387-3-ehabkost@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>