docs: document rng backend path restrictions of older libvirt
docs: document rng backend path restrictions of older libvirt

Commit 67f2b72 removed the path restrictions for rng backend,
along with the documentation of the restriction.

Restore the documentation for users still using older libvirt.

https://bugzilla.redhat.com/show_bug.cgi?id=1074464

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