Ensure your QCOW2 image is not corrupted. Re-import using qemu-img convert :
qemu-img convert -f qcow2 -O qcow2 original_image.qcow2 fresh_image.qcow2 fgtvm64kvmv721fbuild1254fortinetoutkvmqcow2 work
Moving config from FGT VM64-KVM to FGT 80F - Fortinet Community Ensure your QCOW2 image is not corrupted