I have a guest configured with

  spapr-pci-host-bridge
    pci-bridge
      virtio-scsi-pci
        scsi-hd

and SLOF can't seem to be able to boot from the disk:

  SLOF **********************************************************************
  QEMU Starting
   Build Date = May 19 2017 07:43:48
   FW Version = mockbuild@ release 20170303
   Press "s" to enter Open Firmware.

  Populating /vdevice methods
  Populating /vdevice/vty@30000000
  Populating /vdevice/nvram@71000000
  Populating /pci@800000020000000
                       00 0800 (B) : 1b36 0001    pci*
                       01 0800 (D) : 1af4 1004    virtio [ scsi ]
  Populating /pci@800000020000000/pci@1/scsi@1
         SCSI: Looking for devices
            100000000000000 DISK     : "QEMU     QEMU HARDDISK    2.5+"
                       00 1000 (D) : 1af4 1000    virtio [ net ]
                       00 1800 (D) : 1af4 1003    virtio [ serial ]
                       00 2000 (D) : 1af4 1002    unknown-legacy-device*
  No NVRAM common partition, re-initializing...
  Scanning USB
  Using default console: /vdevice/vty@30000000

    Welcome to Open Firmware

    Copyright (c) 2004, 2017 IBM Corporation All rights reserved.
    This program and the accompanying materials are made available
    under the terms of the BSD License available at
    http://www.opensource.org/licenses/bsd-license.php


  Trying to load:  from: /pci@800000020000000/pci-bridge@1/scsi@1/disk@100000000000000 ...
  E3405: No such device

  E3407: Load failed

    Type 'boot' and press return to continue booting the system.
    Type 'reset-all' and press return to reboot the system.


  Ready!
  0 >

If I plug the virtio-scsi-pci straight into pci.0 the guest
boots successfully.

libvirt XML:

  <domain type='kvm'>
    <name>abologna-pegas</name>
    <uuid>51a2947e-015d-4adb-9cae-e5d86e71394c</uuid>
    <memory unit='KiB'>2097152</memory>
    <currentMemory unit='KiB'>2097152</currentMemory>
    <vcpu placement='static'>8</vcpu>
    <os>
      <type arch='ppc64le' machine='pseries-rhel7.4.0'>hvm</type>
      <boot dev='hd'/>
    </os>
    <cpu mode='host-passthrough' check='none'/>
    <clock offset='utc'/>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>destroy</on_crash>
    <devices>
      <emulator>/usr/libexec/qemu-kvm</emulator>
      <disk type='file' device='disk'>
        <driver name='qemu' type='qcow2'/>
        <source file='/var/lib/libvirt/images/abologna-pegas.qcow2'/>
        <target dev='sda' bus='scsi'/>
        <address type='drive' controller='0' bus='0' target='0' unit='0'/>
      </disk>
      <controller type='pci' index='0' model='pci-root'/>
      <controller type='pci' index='1' model='pci-bridge'>
        <model name='pci-bridge'/>
        <target chassisNr='1'/>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
      </controller>
      <controller type='scsi' index='0' model='virtio-scsi'>
        <address type='pci' domain='0x0000' bus='0x01' slot='0x01' function='0x0'/>
      </controller>
      <controller type='usb' index='0' model='none'/>
      <controller type='virtio-serial' index='0'>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
      </controller>
      <interface type='network'>
        <mac address='52:54:00:03:fb:e5'/>
        <source network='default'/>
        <model type='virtio'/>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
      </interface>
      <serial type='pty'>
        <target port='0'/>
        <address type='spapr-vio' reg='0x30000000'/>
      </serial>
      <console type='pty'>
        <target type='serial' port='0'/>
        <address type='spapr-vio' reg='0x30000000'/>
      </console>
      <channel type='unix'>
        <target type='virtio' name='org.qemu.guest_agent.0'/>
        <address type='virtio-serial' controller='0' bus='0' port='1'/>
      </channel>
      <memballoon model='virtio'>
        <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
      </memballoon>
      <panic model='pseries'/>
    </devices>
  </domain>

QEMU command line:

  /usr/libexec/qemu-kvm \
  -name guest=abologna-pegas,debug-threads=on \
  -S \
  -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-6-abologna-pegas/master-key.aes \
  -machine pseries-rhel7.4.0,accel=kvm,usb=off,dump-guest-core=off \
  -cpu host \
  -m 2048 \
  -realtime mlock=off \
  -smp 8,sockets=8,cores=1,threads=1 \
  -uuid 51a2947e-015d-4adb-9cae-e5d86e71394c \
  -display none \
  -no-user-config \
  -nodefaults \
  -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-6-abologna-pegas/monitor.sock,server,nowait \
  -mon chardev=charmonitor,id=monitor,mode=control \
  -rtc base=utc \
  -no-shutdown \
  -boot strict=on \
  -device pci-bridge,chassis_nr=1,id=pci.1,bus=pci.0,addr=0x1 \
  -device virtio-scsi-pci,id=scsi0,bus=pci.1,addr=0x1 \
  -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x3 \
  -drive file=/var/lib/libvirt/images/abologna-pegas.qcow2,format=qcow2,if=none,id=drive-scsi0-0-0-0 \
  -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=1 \
  -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=27 \
  -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:03:fb:e5,bus=pci.0,addr=0x2 \
  -chardev pty,id=charserial0 \
  -device spapr-vty,chardev=charserial0,reg=0x30000000 \
  -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/channel/target/domain-6-abologna-pegas/org.qemu.guest_agent.0,server,nowait \
  -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 \
  -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 \
  -msg timestamp=on

Packages:

  SLOF-20170303-4.git66d250e.el7.noarch
  qemu-kvm-rhev-2.9.0-7.el7.ppc64le