s390x/event-facility: Simplify creation of SCLP event devices

s390x/event-facility: Simplify creation of SCLP event devices

init_event_facility() creates the SCLP events bus with two SCLP event
devices (sclpquiesce and sclp-cpu-hotplug).  It leaves the devices
unrealized.  A comment explains they will be realized "via the bus".

The bus's realize method sclp_events_bus_realize() indeed realizes all
unrealized devices on this bus.  It carries a TODO comment claiming
this "has to be done in common code".  No other bus realize method
realizes its devices.

The common code in question is bus_set_realized(), which has a TODO
comment asking for recursive realization.  It's been asking for years.

The only devices sclp_events_bus_realize() will ever realize are the
two init_event_facility() puts there.

Simplify as follows:

* Make the devices members of the event facility instance struct, just
  like the bus.  object_initialize_child() is simpler than
  object_property_add_child() and object_unref().

* Realize them in the event facility realize method.

This is in line with how such things are done elsewhere.

Cc: Cornelia Huck <cohuck@redhat.com>
Cc: Halil Pasic <pasic@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: qemu-s390x@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200610053247.1583243-54-armbru@redhat.com>