hw/pci: made pci_bus_is_root a PCIBusClass method

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>