Verified on libvirt-1.2.17-13.el7_2.6.x86_64 and PASSED

===================== prepare following script, and having a powered on vm named "vm1" ======================
# cat test.sh
#!/bin/bash

# Purpose: Attach two SCSI disks to a SCSI controller '2'.  The smaller
# controller indices '1' and '0' should be automatically created.

set -x
image_dir="/var/lib/libvirt/images"
vm_name="vm1"
disk_scsi_index="2"

# Create three raw disk images
function create_disk() {
    for i in {b..c}; 
        do 
            qemu-img create -f raw $image_dir/$i.raw 512M; 
        done
}

# This is incrementing the 'unit' (a "2-digit unit number on a bus")
# attribute, for the 'address' sub-element, which describes where the
# device is placed on the virtual bus presented to the guest.
# http://libvirt.org/formatdomain.html#elementsAddress

virsh dumpxml $vm_name | grep scsi

j=1

# Construct a guest XML by attaching two SCSI disks onto the *second* controller
# (and manually increment the 'unit' number)

function attach_device() {
    for i in {b..c}; do
        j=$(expr $j + 1)
        cat << EOF > disk-$i.xml
<disk type='file' device='disk'>
  <source file='$image_dir/$i.raw'/>
  <target dev='sd$i' bus='scsi'/>
  <address type='drive' controller='$disk_scsi_index' model='virtio-scsi' bus='0' target='0' unit='$j'/>
</disk>
EOF

        # Attach the device; it takes effect on next boot
        virsh attach-device $vm_name disk-$i.xml  --live
	virsh dumpxml $vm_name | grep scsi
    done
}

create_disk
attach_device


======================= disk_scsi_index="2" in script ========================

# sh test.sh 
+ image_dir=/var/lib/libvirt/images
+ vm_name=vm1
+ disk_scsi_index=2
+ virsh dumpxml vm1
+ grep scsi
+ j=1
+ create_disk
+ for i in '{b..c}'
+ qemu-img create -f raw /var/lib/libvirt/images/b.raw 512M
Formatting '/var/lib/libvirt/images/b.raw', fmt=raw size=536870912
+ for i in '{b..c}'
+ qemu-img create -f raw /var/lib/libvirt/images/c.raw 512M
Formatting '/var/lib/libvirt/images/c.raw', fmt=raw size=536870912
+ attach_device
+ for i in '{b..c}'
++ expr 1 + 1
+ j=2
+ cat
+ virsh attach-device vm1 disk-b.xml --live
Device attached successfully

+ virsh dumpxml vm1
+ grep scsi
      <target dev='sdb' bus='scsi'/>
      <alias name='scsi2-0-0-2'/>
    <controller type='scsi' index='0'>
      <alias name='scsi0'/>
    <controller type='scsi' index='1'>
      <alias name='scsi1'/>
    <controller type='scsi' index='2'>
      <alias name='scsi2'/>
+ for i in '{b..c}'
++ expr 2 + 1
+ j=3
+ cat
+ virsh attach-device vm1 disk-c.xml --live
Device attached successfully

+ virsh dumpxml vm1
+ grep scsi
      <target dev='sdb' bus='scsi'/>
      <alias name='scsi2-0-0-2'/>
      <target dev='sdc' bus='scsi'/>
      <alias name='scsi2-0-0-3'/>
    <controller type='scsi' index='0'>
      <alias name='scsi0'/>
    <controller type='scsi' index='1'>
      <alias name='scsi1'/>
    <controller type='scsi' index='2'>
      <alias name='scsi2'/>


==============================   disk_scsi_index="10" in script    ===========
# sh test.sh 
+ image_dir=/var/lib/libvirt/images
+ vm_name=vm1
+ disk_scsi_index=10
+ virsh dumpxml vm1
+ grep scsi
+ j=1
+ create_disk
+ for i in '{b..c}'
+ qemu-img create -f raw /var/lib/libvirt/images/b.raw 512M
Formatting '/var/lib/libvirt/images/b.raw', fmt=raw size=536870912
+ for i in '{b..c}'
+ qemu-img create -f raw /var/lib/libvirt/images/c.raw 512M
Formatting '/var/lib/libvirt/images/c.raw', fmt=raw size=536870912
+ attach_device
+ for i in '{b..c}'
++ expr 1 + 1
+ j=2
+ cat
+ virsh attach-device vm1 disk-b.xml --live
Device attached successfully

+ virsh dumpxml vm1
+ grep scsi
      <target dev='sdb' bus='scsi'/>
      <alias name='scsi10-0-0-2'/>
    <controller type='scsi' index='0'>
      <alias name='scsi0'/>
    <controller type='scsi' index='1'>
      <alias name='scsi1'/>
    <controller type='scsi' index='2'>
      <alias name='scsi2'/>
    <controller type='scsi' index='3'>
      <alias name='scsi3'/>
    <controller type='scsi' index='4'>
      <alias name='scsi4'/>
    <controller type='scsi' index='5'>
      <alias name='scsi5'/>
    <controller type='scsi' index='6'>
      <alias name='scsi6'/>
    <controller type='scsi' index='7'>
      <alias name='scsi7'/>
    <controller type='scsi' index='8'>
      <alias name='scsi8'/>
    <controller type='scsi' index='9'>
      <alias name='scsi9'/>
    <controller type='scsi' index='10'>
      <alias name='scsi10'/>
+ for i in '{b..c}'
++ expr 2 + 1
+ j=3
+ cat
+ virsh attach-device vm1 disk-c.xml --live
Device attached successfully

+ virsh dumpxml vm1
+ grep scsi
      <target dev='sdb' bus='scsi'/>
      <alias name='scsi10-0-0-2'/>
      <target dev='sdc' bus='scsi'/>
      <alias name='scsi10-0-0-3'/>
    <controller type='scsi' index='0'>
      <alias name='scsi0'/>
    <controller type='scsi' index='1'>
      <alias name='scsi1'/>
    <controller type='scsi' index='2'>
      <alias name='scsi2'/>
    <controller type='scsi' index='3'>
      <alias name='scsi3'/>
    <controller type='scsi' index='4'>
      <alias name='scsi4'/>
    <controller type='scsi' index='5'>
      <alias name='scsi5'/>
    <controller type='scsi' index='6'>
      <alias name='scsi6'/>
    <controller type='scsi' index='7'>
      <alias name='scsi7'/>
    <controller type='scsi' index='8'>
      <alias name='scsi8'/>
    <controller type='scsi' index='9'>
      <alias name='scsi9'/>
    <controller type='scsi' index='10'>
      <alias name='scsi10'/>

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2016-2688.html