resize_claim lazy-loads at least 3 joined fields in separate DB calls

During a resize_claim the ResourceTracker lazy-loads in 3 separate calls to the DB (over RPC) these 3 fields:

    b"2019-11-20 16:13:29,521 DEBUG [nova.objects.instance] Lazy-loading 'pci_requests' on Instance uuid c0fdac69-b360-4526-917e-16fb018cb8a3"
    b"2019-11-20 16:13:29,525 DEBUG [nova.objects.instance] Lazy-loading 'resources' on Instance uuid c0fdac69-b360-4526-917e-16fb018cb8a3"
    b"2019-11-20 16:13:29,527 DEBUG [nova.objects.instance] Lazy-loading 'pci_devices' on Instance uuid c0fdac69-b360-4526-917e-16fb018cb8a3"

It seems we should be able to collapse that into a single DB call to load the necessary fields in a single call.

We could add a new extra_attrs kwarg to the Instance.refresh method so we can keep using the same instance we have in memory (and is shared by the ComputeManager method calling the resize_claim) or we could add a new load_if_not_present() method to the Instance object. I'm not sure if there are pros/cons either way on using refresh or adding a new method.