pci: Validate interfaces on base_class_init

Make sure we don't forget to add the Conventional PCI or PCI
Express interface names on PCI device classes in the future.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Revieed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
