Even then, I suspect sooner or later machines other than PC and Q35
will be versioned. At that point we'll probably want QEMU-global
compat_props that automatically apply to all machines, even if a type
is not missing. I think we should already approximate this behavior
by allowing machine .compat_props where the type doesn't exist.
If I am reading this correctly, this is asking that .compat_props to not
be reported. This currently happens because of not_used being false
in all C99 struct initializations.