api: Adjust comment for virDomainAddIOThread
api: Adjust comment for virDomainAddIOThread

The comment for the function indicated that iothread_id had to be
a positive non-zero value; however, that wasn't checked - that is
a value of 0 is/was allowed by the API and was left up to the
hypervisor to reject the value.

More than likely this nuance was missed during the many "adjustments"
to the API in the review phase.

crobinso/libvirt,datto/libvirt,zippy2/libvirt,crobinso/libvirt,eskultety/libvirt,datto/libvirt,jardasgit/libvirt,eskultety/libvirt,rlaager/libvirt,fabianfreyer/libvirt,agx/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,taget/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,eskultety/libvirt,rlaager/libvirt,taget/libvirt,crobinso/libvirt,fabianfreyer/libvirt,agx/libvirt,rlaager/libvirt,andreabolognani/libvirt,jardasgit/libvirt,agx/libvirt,libvirt/libvirt,eskultety/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,taget/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,datto/libvirt,olafhering/libvirt,datto/libvirt,zippy2/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,taget/libvirt,agx/libvirt,libvirt/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,jardasgit/libvirt,jfehlig/libvirt,rlaager/libvirt,fabianfreyer/libvirt,agx/libvirt,libvirt/libvirt,eskultety/libvirt,jfehlig/libvirt,crobinso/libvirt,taget/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,rlaager/libvirt,olafhering/libvirt,libvirt/libvirt,datto/libvirt,andreabolognani/libvirt,zippy2/libvirt,VenkatDatta/libvirt