conf: Check for attach disk usage of iothread=0
conf: Check for attach disk usage of iothread=0

Since iothreadid = 0 is invalid, we need to check for it when attempting
to add a disk; otherwise, someone would think/believe their attempt to
add an IOThread to the disk would succeed. Luckily other code ignored
things when ->iothread == 0...

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