pci: Add pci_dev_bus_num() helper

pci: Add pci_dev_bus_num() helper

A fair proportion of the users of pci_bus_num() want to get the bus
number on a specific device, so first have to look up the bus from the
device then call it.  This adds a helper to do that (since we're going
to make looking up the bus slightly more verbose).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>