pci: Add root bus argument to pci_get_bus_devfn()

pci: Add root bus argument to pci_get_bus_devfn()

pci_get_bus_devfn() interprets a full PCI address string to give a PCIBus *
and device/function number within that bus.  Currently it assumes it is
working on an address under the primary PCI root bus.  This patch extends
it to allow the caller to specify a root bus.  This might seem a little odd
since the supplied address can (theoretically) include a PCI domain number.
However, attempting to use a non-zero domain number there is currently an
error, so that shouldn't really cause problems.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>