Description of problem:
Creating external disk snapshot for a guest which has two disks with the same prefix name，the disks become the same name in xml

Version-Release number of selected component (if applicable):
libvirt-1.2.17-13.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1)prepare a guest having two disk,which have the same prefix name
[root@localhost ~]# virsh domblklist fs
Target     Source
------------------------------------------------
hda        /mnt/fs/f.qcow2
vda        /mnt/fs/f.qcow2-bak


2)create external disk snapshot
[root@localhost ~]# virsh snapshot-create-as fs test --disk-only
Domain snapshot test created

3)look at the xml file
[root@localhost ~]# virsh dumpxml fs | grep disk -A 10
    <suspend-to-disk enabled='no'/>
  </pm>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/mnt/fs/f.test'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>
       ***   <source file='/mnt/fs/f.qcow2-bak'/> ***
        <backingStore/>
      </backingStore>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/mnt/fs/f.test'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>
     ***     <source file='/mnt/fs/f.qcow2-bak'/> ***
        <backingStore/>
      </backingStore>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </disk>

Actual results:
The names of the two disks become the same in the xml file.


Expected results:


Additional info: