PCI: Export pci_iov_virtfn_bus() and pci_iov_virtfn_devfn()

On PowerNV, some resource reservation is needed for SR-IOV VFs that don't
exist at the bootup stage.  To do the match between resources and VFs, the
code need to get the VF's BDF in advance.

Rename virtfn_bus() and virtfn_devfn() to pci_iov_virtfn_bus() and
pci_iov_virtfn_devfn() and export them.

[bhelgaas: changelog, make "busnr" int]
Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>