pci: stop the temporary "belowroot" mess

v2.0 temporarily attached all PCI objects to the root so that
backends adding OS objects inside those PCI could easily find them.
PCI were later moved to their right location.

Now directly attach PCI to their right location and use an internal
table to remember the locality of PCI buses.

For now this is only used during discovery, but we may also use it
later when applications lookup PCI by bus IDs too (instead of traversing
the list of PCI stuff).
It will require an internal/shmem ABI change so defer that for when
we'll have more reasons to change that ABI.

Fixes #249

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>