Add fw_cfg_arch_key_name() which returns the name of
an architecture-specific key.
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Message-Id: <20190422195020.1494-3-philmd@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>