fw_cfg QOM interface is required by system emulation and
qemu-storage-daemon. User-mode emulation doesn't need it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220111184309.28637-3-f4bug@amsat.org>