add DMABUF support for zero-copy vhost-user-gpu display

Add DMABUF consumer support to display backend for zero-copy rendering with vhost-user-gpu + virglrenderer backend.

Tested with both backends:

• gfxstream (software rendering via framebuffer path)
• virglrenderer (zero-copy DMABUF path)

Testing

Start the vhost-device-gpu backend with virglrenderer:

vhost-device-gpu --socket-path /tmp/gpu.socket --gpu-mode virglrenderer

Start gui_vm with vhost-user GPU:

./examples/target/release/gui_vm --root-dir /path/to/rootfs --display 1024x768 --vhost-user-gpu /tmp/gpu.socket /bin/bash

Then run benchmarks:

• kmscube
• glmark2-drm