https://bugzilla.redhat.com/show_bug.cgi?id=1135491

More or less a virtual copy of the existing virDomainVcpuPin{Add|Del} API's.

NB: The IOThreads implementation "reused" the virDomainVcpuPinDefPtr
since it provided everything necessary - an "id" and a "map" for each
thread id configured.