Add ThreadID as a new CPU attribute

[connorkuehl]

Reposting #149 with authorship intact so this can run against CI.

This change is contemporaneous with the version of the QAPI that's currently generated against, but I suspect it will change in future revisions because I can't find any reference to query-cpus in modern QAPI references, only query-cpus-fast. I'm only pointing this out because this struct won't be forwards compatible with that CPU info struct, for example, query-cpus-fast returns a JSON object where the key is thread-id and not thread_id.

But, as stated above, this is correct for the version of QAPI that's been ossified here.