conf: Error out if iothread id is missing in iothreadpin
conf: Error out if iothread id is missing in iothreadpin

Defining a domain with the following config:

<domain ...>
  ...
  <iothreads>1</iothreads>
  <cputune>
    <iothreadpin cpuset='1'/>

will result in the following config formatted back:
<domain type='kvm'>
  ...
  <iothreads>1</iothreads>
  <cputune>
    <iothreadpin iothread='0' cpuset='1'/>

After restart the VM would vanish. Since our schema requires the
@iothread field to be present in <iothreadpin> make it required by the
code too.

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