conf: use #define instead of literal for highest slot in upstream port

Every other maxSlot was either set to 0 or to
VIR_PCI_ADDRESS_SLOT_LAST, but this one was for some reason set to the
literal value 31 (which is the same as VIR_PCI_ADDRESS_SLOT_LAST).
This makes them all consistent.

src/conf/domain_addr.c

@@ -212,7 +212,7 @@ virDomainPCIAddressBusSetModel(virDomainPCIAddressBusPtr bus,
         /* 31 slots, can only accept pcie-switch-port, no hotplug */
         bus->flags = VIR_PCI_CONNECT_TYPE_PCIE_SWITCH;
         bus->minSlot = 0;
-        bus->maxSlot = 31;
+        bus->maxSlot = VIR_PCI_ADDRESS_SLOT_LAST;
         break;
     default:
         virReportError(VIR_ERR_INTERNAL_ERROR,