virtio-gpu.c: add resource_destroy class method

virtio-gpu.c: add resource_destroy class method

When destroying/unrefing resources, devices such as virtio-gpu-rutabaga
need to do their own bookkeeping (free rutabaga resources that are
associated with the virtio_gpu_simple_resource).

This commit adds a class method so that virtio-gpu-rutabaga can override
it in the next commit.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-Id: <b0a86630c4d601f3a269fd7e08cfefc13bd4e219.1706626470.git.manos.pitsidianakis@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>