virtio/vhost-user: add multi-scanout support for GPU devices

Allow vhost-user-gpu to handle multiple displays by:

• Changing gpu_display_info from Option to Vec
• Supporting multiple scanout_id values (0..N instead of hardcoded 0)
• Returning appropriate EDID for each configured display (empty EDID for scanouts beyond configured displays)
• Passing all displays from device_builders instead of just first

Multiple displays can be configured via repeated --display flags. Tested with 2 displays, both scanouts correctly enumerated and functional in the guest.