qemu: clarify error message for index 0 PIIX3 USB controller
qemu: clarify error message for index 0 PIIX3 USB controller

The address is restricted to 0:0:1.2 only for the piix3-uhci controller
at index 0.

https://bugzilla.redhat.com/show_bug.cgi?id=1460602

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