The TYPE_PNV_PHB3_ROOT_BUS name is used as the default bus name when
the dev has no 'id'. However, pnv-phb3-root-bus is a bit too long to be
used as a bus name.

Most common QEMU buses and PCI controllers are named based on their bus
type (e.g. pSeries spapr-pci-host-bridge is called 'pci'). The most
common name for a PCIE bus controller in QEMU is 'pcie'. Naming it
'pcie' would break the documented use of the pnv-phb3 device, since
'pcie.0' would now refer to the root bus instead of the first root port.

There's nothing particularly wrong with the 'root-bus' name used before,
aside from the fact that 'root-bus' is being used for pnv-phb3 and
pnv-phb4 created buses, which is not quite correct since these buses
aren't implemented the same way in QEMU - you can't plug a
pnv-phb4-root-port into a pnv-phb3 root bus, for example.

This patch renames it as 'pnv-phb3-root', which is a compromise between
the existing and the previously used name. Creating 3 phbs without ID
will result in an "info qtree" output similar to this:

bus: main-system-bus
  type System
  dev: pnv-phb3, id ""
    index = 2 (0x2)
    chip-id = 0 (0x0)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb3-root.2
      type pnv-phb3-root
(...)
  dev: pnv-phb3, id ""
    index = 1 (0x1)
    chip-id = 0 (0x0)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb3-root.1
      type pnv-phb3-root
(...)
  dev: pnv-phb3, id ""
    index = 0 (0x0)
    chip-id = 0 (0x0)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb3-root.0
      type pnv-phb3-root

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220105212338.49899-11-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>