qemu: command: Move disk index validation closer to usage
qemu: command: Move disk index validation closer to usage

The disk index validation is used only in very specific cases and does
not need to be performed otherwise. Move it out of the global check into
the usage place.

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