Implement fw_cfg_arch_key_name(), which returns the name of a
ppc-specific key.

The fw_cfg device is used by the machine using OpenBIOS:
- 40p
- mac99 (oldworld)
- g3beige (newworld)

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190422195020.1494-6-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>