Add an entry to cover firmware.json (see commit 3a0adfc9:
schema that describes the different uses and properties of
virtual machine firmware).

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Kashyap Chamarthy <kchamart@redhat.com>
Cc: Daniel P. Berrange <berrange@redhat.com>
Suggested-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200703183450.32398-1-philmd@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>