virtio-gpu: fix resource leak in virgl_cmd_resource_unref

When the guest sends VIRTIO_GPU_CMD_RESOURCE_UNREF without detaching the
backing storage beforehand (VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING)
we'll leak memory.

This patch fixes it for 3d mode, simliar to the 2d mode fix in commit
"b8e2392 virtio-gpu: call cleanup mapping function in resource destroy".

Reported-by: 李强 <liqiang6-s@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1485167210-4757-1-git-send-email-kraxel@redhat.com
