pci_bridge: user-friendly default bus name

For a pci bridge device, if we don't override
the name with custom code, the bus will be addressed as
<id>.0, where id is the id specified by the user.

Since PCI Bridge devices have a single bus each, we don't need
the index: address the bus using the parent device name.
This is better since this way users don't care about
our internal bus/device distinctions.

As far as I could see, we only have built-in
bridges at this point which always override the
name. So this change will only affect ioh3420.c.

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