docs: domain: Show canonical pvspinlock XML
docs: domain: Show canonical pvspinlock XML

The example pvspinlock XML is:

  <pvspinlock/>

While this is accepted by libvirt and works correctly, it's currently
always output as a tristate like

  <pvspinlock state='on'/>

So document that format instead

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