With the new approach we are actually able to correctly detect node
names for the two instances of the same backing file.

Test images were created as:
qemu-img create -f qcow2 /var/lib/libvirt/images/base.qcow2 10M
qemu-img create -f qcow2 \
   -o "backing_fmt=qcow2,backing_file=/var/lib/libvirt/images/base.qcow2 \
   /var/lib/libvirt/images/a.qcow2
qemu-img create -f qcow2 \
   -o "backing_fmt=qcow2,backing_file=/var/lib/libvirt/images/base.qcow2 \
   /var/lib/libvirt/images/b.qcow2

and then used for two separate disks.
Reviewed-by: NEric Blake <eblake@redhat.com>