device_add plugs devices into suitable bus.  For "real" buses, that
actually connects the device.  For sysbus, the connections need to be
made separately, and device_add can't do that.  The device would be
left unconnected, and could not possibly work.

Quite a few, but not all sysbus devices already set
cannot_instantiate_with_device_add_yet in their class init function.

Set it in their abstract base's class init function
sysbus_device_class_init(), and remove the now redundant assignments
from device class init functions.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMarcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>