Moving to POST:

http://post-office.corp.redhat.com/archives/rhvirt-patches/2016-January/msg00530.html

Reproduce this bug with:
libvirt-1.2.17-13.el7_2.2.x86_64

Steps:
1.In esx 6.0,find a guest,modify it's vmx file as below:
From:
ide1:0.deviceType = "cdrom-image"
ide1:0.fileName =  "/vmfs/volumes/f0a07907-3759bde7/en_win_srv_2003_r2_enterprise_with_sp2_cd1_X13-05460.iso"                        ide1:0.present = "TRUE"

to:
ide1:0.deviceType = "cdrom-image"
ide1:0.fileName = "emptyBackingString"    ----> modify this value                                                     
ide1:0.present = "TRUE"

2.Save the above change,from vsphere client,I can see in "CD/DVD driver 1" shows iso File path as "[]".

3.Use virsh to connect the esx server and dumpxml the xml of the above guest.
# virsh -c esx://$esx/?no_verify=1
Enter username for $esx [root]: 
Enter root's password for $esx: 

virsh # list --all
 Id    Name                           State
----------------------------------------------------
 1     win2012R2-vcenter              running
 33    iscsi-test                     running
 43    VMware-vcenter-server-app      running
 -     app_vcenter600                 shut off
 -     Auto-esx6.0-win7-efi           shut off
 -     bug1260689-win2008R2-GPO-Macfee shut off
 -     esx6.0-rhel5.11-i386           shut off
 -     esx6.0-rhel5.11-x86_64         shut off
 -     esx6.0-rhel6.7-i386            shut off
 -     esx6.0-rhel6.7-x86_64          shut off
 -     esx6.0-rhel7.2-x86_64          shut off
 -     esx6.0-win2003-i386            shut off
 -     esx6.0-win2003-x86_64          shut off
 -     esx6.0-win2008-i386            shut off
 -     esx6.0-win2008-x86_64          shut off
 -     test-juzhou                    shut off


virsh # dumpxml test-juzhou
error: internal error: Invalid or not yet handled value 'emptyBackingString' for VMX entry 'ide1:0.fileName' for device type 'cdrom-image'

Verify the bug with:
libvirt-1.2.17-13.el7_2.3.x86_64

In the steps as above,in step3,the result shows as below:
virsh # dumpxml test-juzhou
<domain type='vmware'>
  <name>test-juzhou</name>
  <uuid>423f34eb-61a7-72db-5a24-2f6088db68bd</uuid>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <cputune>
    <shares>1000</shares>
  </cputune>
  <os>
    <type arch='x86_64'>hvm</type>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <source file='[datastore1] test-juzhou/test-juzhou.vmdk'/>
      <target dev='sda' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <source file='[datastore1] test-juzhou/test-juzhou_1.vmdk'/>
      <target dev='sdb' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
    <disk type='file' device='cdrom'>
      <target dev='hdc' bus='ide'/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
    <controller type='scsi' index='0' model='vmpvscsi'/>
    <controller type='ide' index='0'/>
    <interface type='bridge'>
      <mac address='00:50:56:bf:3e:50'/>
      <source bridge='VM Network'/>
      <model type='vmxnet3'/>
    </interface>
    <video>
      <model type='vmvga' vram='8192'/>
    </video>
  </devices>
</domain>

For the guest,the xml file can be got successfully via virsh dumpxml,so the bug is fixed and move the bug to VERIFIED.

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-2016-0220.html