Restricting system_wakeup/system_reset/system_powerdown to
machine.json pulls slightly less QAPI-generated code into
user-mode and tools.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201012121536.3381997-3-philmd@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>