I have used

virsh snapshot-create-as <VM name> <snapshot name> "<snapshot description>" --diskspec "vda,snapshot=external,file=/path/to/external-snapshot" --disk-only --atomic

to create an external snapshot of a running VM.  I followed it with

virsh blockpull <VM name> --path /path/to/external-snapshot

and monitored it until done.  I confirmed it with

qemu-img info /path/to/external-snapshot

which shows no backing store.

However, when I do

grep {/etc,/run}/libvirt/qemu/<VM name>.xml

the xml for the VM still shows a backing store in the /run/... definition (but not in the /etc/... definition, my understanding is that the /etc/... definition won't be updated until the VM is shut down):

      <disk type='file' device='disk'>
        <driver name='qemu' type='qcow2'/>
        <source file='/var/lib/libvirt/images/external-snapshot.qcow2'/>
        <backingStore type='file' index='1'>
          <format type='qcow2'/>
          <source file='/var/lib/libvirt/images/original-image-file.qcow'/>
          <backingStore/>
        </backingStore>
        <target dev='hda' bus='ide'/>
        <alias name='ide0-0-0'/>
        <address type='drive' controller='0' bus='0' target='0' unit='0'/>
      </disk>

This is using libvirtd version 1.3.1 and qemu-img version 2.5.0.

How do I resolve this situation?  I don't want to have to rely on a backing store permanently.  Thanks for any help or pointers.