virtio-gpu: define a new command to get p2pdma_distance

[julizhan]

We are trying to implement dGPU prime feature in guest which will let virtio-iGPU read rendered data of passthrough dGPU. Before that, virtio gpu driver needs to get p2pdma_distance to check if P2P DMA transaction is possible or not.

To implement getting p2pdma_distance, QEMU needs to handle the command from guest with virtual pci notations of two PCI devices and send it to host kernel and return host physical distance back to guest.

So we need to define a new command follow the suggestion in https://lore.kernel.org/all/20241207105537.542441-4-julia.zhang@amd.com/

Latest proposal: https://lore.kernel.org/all/20250114101612.88304-1-julia.zhang@amd.com/