Make pci_requests a proper field on Instance object

When we added the pci_requests object, we didn't plumb it all the way into the
Instance object because of time constraints. This finishes that work so that it
behaves properly and doesn't require an additional DB lookup every time we need to
pull it based on an instance.

Partial-Bug: #1368260
Change-Id: I134441c8288d5dbdc0bac6163cb2affc9f0af762
Merge "Make pci_requests a proper field on Instance object"