spapr: Don't use bus number for building DRC ids

DRC ids are more or less arbitrary, as long as they're consistent.  For
PCI, we notionally build them from the phb's index along with PCI bus
number, slot and function number.

Using bus number is broken, however, because it can change if the guest
re-enumerates the PCI topology for whatever reason (e.g. due to hotplug
of a bridge, which we don't support yet but want to).

Fortunately, there's an alternative.  Bridges are required to have a unique
non-zero "chassis number" that we can use instead.  Adjust the code to
use that instead.

This looks like it would introduce a guest visible breaking change, but
in fact it does not because we don't yet ever use non-zero bus numbers.
Both chassis and bus number are always 0 for the root bus, so there's no
change for the existing cases.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

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