conf: validate that PCI controller index is < 256
conf: validate that PCI controller index is < 256

This is the maximum for many reasons, for starters because index ==
bus number, and a controller's bus number is 8 bits.

This incidentally resolves: https://bugzilla.redhat.com/1329090

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