pci: fix migration path for devices behind bridges

pci: fix migration path for devices behind bridges

The device path used for migration is currently broken for
for all devices behind a nested bridge.

Replace this by a hierarchical list of slot/function numbers, walking
the path from root down to device. Add :00 after the domain number
so that if there are no nested bridges, this is compatible
with what we have now.

Note: as pointed out by Gleb, using openfirmware paths
might be cleaner, doing this would break compatibility though,
and the IDs used are not guest or user visible at all,
so breaking the compatibility is probably not worth it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>