pnv_phb3.c: do not set 'root-bus' as bus name

All pnv-phb3-root-bus buses are being created as 'root-bus'. This
makes it impossible to, for example, add a pnv-phb3-root-port in
a specific root bus, since they all have the same name. By default
the device will be parented by the pnv-phb3 device that precedeced it in
the QEMU command line.

Moreover, this doesn't all for custom bus naming. Libvirt, for instance,
likes to name these buses as 'pcie.N', where 'N' is the index value of
the controller in the domain XML, by using the 'id' command line
attribute. At this moment this is also being ignored - the created root
bus will always be named 'root-bus'.

This patch fixes both scenarios by removing the 'root-bus' name from the
pci_register_root_bus() call. If an "id" is provided, use that.
Otherwise use 'NULL' as bus name. The 'NULL' value will be handled in
qbus_init_internal() and it will defaulted as lowercase bus type + the
global bus_id value.

After this path we can define the bus name by using the 'id' attribute:

qemu-system-ppc64 -m 4G -machine powernv8,accel=tcg \
    -device pnv-phb3,chip-id=0,index=1,id=pcie.0

  dev: pnv-phb3, id "pcie.0"
    index = 1 (0x1)
    chip-id = 0 (0x0)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pcie.0
      type pnv-phb3-root-bus

And without an 'id' we will have the following default:

qemu-system-ppc64 -m 4G -machine powernv8,accel=tcg \
    -device pnv-phb3,chip-id=0,index=1

  dev: pnv-phb3, id ""
    index = 1 (0x1)
    chip-id = 0 (0x0)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb3-root-bus.0
      type pnv-phb3-root-bus

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211228193806.1198496-3-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/pci-host/pnv_phb3.c

@@ -1045,7 +1045,8 @@ static void pnv_phb3_realize(DeviceState *dev, Error **errp)
     memory_region_init(&phb->pci_mmio, OBJECT(phb), "pci-mmio",
                        PCI_MMIO_TOTAL_SIZE);
 
-    pci->bus = pci_register_root_bus(dev, "root-bus",
+    pci->bus = pci_register_root_bus(dev,
+                                     dev->id ? dev->id : NULL,
                                      pnv_phb3_set_irq, pnv_phb3_map_irq, phb,
                                      &phb->pci_mmio, &phb->pci_io,
                                      0, 4, TYPE_PNV_PHB3_ROOT_BUS);