pci: fix error message for express slots

PCI Express downstream slot has a single PCI slot
behind it, using PCI_DEVFN(PCI_SLOT(devfn), 0)
does not give you function 0 in cases such as ARI
as well as some error cases.

This is exactly what we are hitting:
   $ qemu-system-x86_64 -machine q35 -readconfig docs/q35-chipset.cfg
-monitor stdio
   (qemu) device_add e1000e,bus=ich9-pcie-port-4,addr=00
   (qemu) device_add e1000e,bus=ich9-pcie-port-4,addr=08
   Segmentation fault (core dumped)

The fix is to use the pci_get_function_0 API.

Cc: qemu-stable@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Tested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
(cherry picked from commit d93ddfb1f8)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/pci/pci.c

@@ -982,8 +982,8 @@ static PCIDevice *do_pci_register_device(PCIDevice *pci_dev, PCIBus *bus,
                pci_get_function_0(pci_dev)) {
         error_setg(errp, "PCI: slot %d function 0 already ocuppied by %s,"
                    " new func %s cannot be exposed to guest.",
-                   PCI_SLOT(devfn),
-                   bus->devices[PCI_DEVFN(PCI_SLOT(devfn), 0)]->name,
+                   PCI_SLOT(pci_get_function_0(pci_dev)->devfn),
+                   pci_get_function_0(pci_dev)->name,
                    name);
 
        return NULL;