hw/display: introduce virtio-dmabuf

hw/display: introduce virtio-dmabuf

This API manages objects (in this iteration,
dmabuf fds) that can be shared along different
virtio devices, associated to a UUID.

The API allows the different devices to add,
remove and/or retrieve the objects by simply
invoking the public functions that reside in the
virtio-dmabuf file.

For vhost backends, the API stores the pointer
to the backend holding the object.

Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
Message-Id: <20231002065706.94707-3-aesteve@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>