hw/pci: made pci_bus_is_root a PCIBusClass method Refactoring it as a method of PCIBusClass will allow different implementations for subclasses. Removed the assumption that the root bus does not have a parent device because is specific only to the default class implementation. Signed-off-by: Marcel Apfelbaum <marcel@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Laszlo Ersek <lersek@redhat.com>