qemu-img convert malfunction on macOS

On macOS 10.13.6, `qemu-img convert` failed to convert a qcow2 into a new qcow2 (for the purpose of shrinking the image).

A 50GB (3.7GB allocated) qcow2 image was used as source. The qemu-img convert output was a 3.4MB file. 

qemu-img from HomeBrew were tested. Both 2.11.1 and 3.1.0_1 failed to convert a qcow2 image.