The filename fgtvm64kvmv747mbuild2731fortinetoutkvmqcow2 can be broken down as follows:
Do not download this unless you can verify the SHA256 checksum from an official Fortinet support portal.
fortios.qcow2: The primary virtual disk image containing the operating system.
Locate the Build: Look for the file matching Build 2731 for the KVM platform. 2. Image Specifications Version: 7.4.7 Build: 2731 Platform: KVM / QEMU