Restrict hotplug to system emulation, add stubs for the other uses.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20211028150521.1973821-5-philmd@redhat.com>