If you have a direct download link from your support account, you can use:
sudo virt-install \ --name fortigate-v6 \ --ram 4096 \ --vcpus 2 \ --disk path=/var/lib/libvirt/images/fortigate-v6.qcow2,format=qcow2,bus=virtio \ --import \ --network bridge=br-wan,model=virtio \ --network bridge=br-lan,model=virtio \ --os-variant generic \ --graphics vnc \ --console pty,target_type=serial \ --noautoconsole Fgt-vm64-kvm-v6-build1010-fortinet.out.kvm.qcow2 Download