This change has the same motivation as the one done for pnv-phb3-root-bus
buses previously. Defaulting every bus to 'root-bus' makes it impossible to attach
root ports to specific buses and it doesn't allow for custom bus
naming because we're ignoring the 'id' value when registering the root
bus.

After this patch, creating pnv-phb4 devices with 'id' being set will
result in the following qtree:

qemu-system-ppc64 -m 4G -machine powernv9,accel=tcg \
   -device pnv-phb4,chip-id=0,index=0,id=pcie.0 \
   -device pnv-phb4,chip-id=1,index=4,id=pcie.1

bus: main-system-bus
  type System
  dev: pnv-phb4, id "pcie.1"
    index = 4 (0x4)
    chip-id = 1 (0x1)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pcie.1
      type pnv-phb4-root-bus
  dev: pnv-phb4, id "pcie.0"
    index = 0 (0x0)
    chip-id = 0 (0x0)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pcie.0
      type pnv-phb4-root-bus

And without setting any ids:

qemu-system-ppc64 -m 4G -machine powernv9,accel=tcg \
   -device pnv-phb4,chip-id=0,index=0,id=pcie.0 \
   -device pnv-phb4,chip-id=1,index=4,id=pcie.1

bus: main-system-bus
  type System
  dev: pnv-phb4, id ""
    index = 4 (0x4)
    chip-id = 1 (0x1)
    version = 704374636546 (0xa400000002)
    device-id = 1217 (0x4c1)
    x-config-reg-migration-enabled = true
    bypass-iommu = false
    bus: pnv-phb4-root-bus.1
      type pnv-phb4-root-bus
  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-bus.0
      type pnv-phb4-root-bus

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20211228193806.1198496-17-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>