Some recent error handling cleanups unveiled issues with our support of
PCI bridges:

1) QEMU aborts when using non-standard PCI bridge types,
   unveiled by commit 7ef1553d "spapr_pci: Drop some dead error handling"

$ qemu-system-ppc64 -M pseries -device pcie-pci-bridge
Unexpected error in object_property_find() at qom/object.c:1240:
qemu-system-ppc64: -device pcie-pci-bridge: Property '.chassis_nr' not found
Aborted (core dumped)

This happens because we assume all PCI bridge types to have a "chassis_nr"
property. This property only exists with the standard PCI bridge type
"pci-bridge" actually. We could possibly revert 7ef1553d but it seems
much simpler to check the presence of "chassis_nr" earlier.

2) QEMU abort if same "chassis_nr" value is used several times,
   unveiled by commit d2623129 "qom: Drop parameter @errp of
   object_property_add() & friends"

$ qemu-system-ppc64 -M pseries -device pci-bridge,chassis_nr=1 \
                        -device pci-bridge,chassis_nr=1
Unexpected error in object_property_try_add() at qom/object.c:1167:
qemu-system-ppc64: -device pci-bridge,chassis_nr=1: attempt to add duplicate property '40000100' to object (type 'container')
Aborted (core dumped)

This happens because we assume that "chassis_nr" values are unique, but
nobody enforces that and we end up generating duplicate DRC ids. The PCI
code doesn't really care for duplicate "chassis_nr" properties since it
is only used to initialize the "Chassis Number Register" of the bridge,
with no functional impact on QEMU. So, even if passing the same value
several times might look weird, it never broke anything before, so
I guess we don't necessarily want to enforce strict checking in the PCI
code now.

Workaround both issues in the PAPR code: check that the bridge has a
unique and non null "chassis_nr" when plugging it into its parent bus.

Fixes: 05929a6c ("spapr: Don't use bus number for building DRC ids")
Fixes: 7ef1553d ("spapr_pci: Drop some dead error handling")
Fixes: d2623129 ("qom: Drop parameter @errp of object_property_add() & friends")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159431476748.407044.16711294833569014964.stgit@bahia.lan>
[dwg: Move check slightly to a better place]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>