Build 2731 requires a license file. Upload it via:
qemu-system-x86_64 -enable-kvm -m 2048 -vcpus 2 -drive file=fgtvm64kvmv747mbuild2731.qcow2,format=qcow2 -device virtio-net-pci,netdev=net0 -netdev user,id=net0,hostfwd=tcp::10022-:22 fgtvm64kvmv747mbuild2731fortinetoutkvmqcow2 new
Build 2731 may require a certain libvirt or qemu-kvm version. Check compatibility: Build 2731 requires a license file