pnv_phb4.c: change TYPE_PNV_PHB4_ROOT_BUS name

Similar to what was happening with pnv-phb3 buses,
TYPE_PNV_PHB4_ROOT_BUS set to "pnv-phb4-root-bus" is a bit too long for
a default root bus name. The usual default name for theses buses in QEMU
are 'pcie', but we want to make a distinction between pnv-phb4 buses and
other PCIE buses, at least as far as default name goes, because not all
PCIE devices are attachable to a pnv-phb4 root-bus type.

Changing the default to 'pnv-phb4-root' allow us to have a shorter name
while making this bus distinct, and the user can always set its own bus
naming via the "id" attribute anyway.

This is the 'info qtree' output after this change, using a powernv9
domain with 2 sockets and default settings enabled:

qemu-system-ppc64 -m 4G -machine powernv9,accel=tcg \
     -smp 2,sockets=2,cores=1,threads=1

  dev: pnv-phb4, id ""
    index = 5 (0x5)
    chip-id = 1 (0x1)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb4-root.11
      type pnv-phb4-root
      dev: pnv-phb4-root-port, id ""
(...)
  dev: pnv-phb4, id ""
    index = 0 (0x0)
    chip-id = 1 (0x1)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb4-root.6
      type pnv-phb4-root
      dev: pnv-phb4-root-port, id ""
(..)
  dev: pnv-phb4, id ""
    index = 5 (0x5)
    chip-id = 0 (0x0)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb4-root.5
      type pnv-phb4-root
      dev: pnv-phb4-root-port, id ""
(...)
  dev: pnv-phb4, id ""
    index = 0 (0x0)
    chip-id = 0 (0x0)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb4-root.0
      type pnv-phb4-root
      dev: pnv-phb4-root-port, id ""

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220110143346.455901-11-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>