I have a qcow2 image A,

     a qcow2 delta image B whose backing file is A,

     and a qcow2 image C whose backing file is B.

     Now I want to merge the B and C into a qcow2 image D whose backing file is A.

     I have tried to use qemu-img to solve it, but still didn't get positive solutions.

virsh blockpull --domain vmname --path /var/lib/libvirt/images/c.qcow2

cp C D
qemu-img rebase -b A D