refer to qemu vfio-user spec

[jlevon]

We should figure out how to refer to qemu's copy of vfio-user spec, and only document our yet to be merged extensions (if relevant)

[jlevon]

fixed via #817

[jlevon]

https://lore.kernel.org/qemu-devel/20251010102453.711072-1-john.levon@nutanix.com/T/#u