Not sure if it's relevant anymore but I do have some virtual DS918+ running on SUSE KVM hosts (openSUSE Leap and SLES 15 SP1) for testing purposes.
As always, you will need to modify the synoboot file to match the MAC address. Then create a new VM using virt-manager following these steps:
1. Click to create a new VM, select "Import existing disk image"
2. On the next screen select the modified synoboot-ds918.img, as an OS, I normally just select "Generic default"
3. Increase memory and CPU to what you required (as a minimum I always select 4096 MB and 4 cores to match the DS918+)
4. On the next screen, change the name and check your network selection - IMPORTANT: Enable the "Customize configuration before install" option and click Finish.
5. Change/make sure that the "synoboot-ds918.img" is set to "USB" under Advanced Options. Under "Performance options", change the "Cache mode" to "none" and "IO mode" to "native"
6. Click "Add Hardware" to add another disc (ie. your data disk), make sure to select "SATA" under "Bus type" and again under "Performance options", change the "Cache mode" to "none" and "IO mode" to "native"
7. Double check the network card is "e1000" and that the MAC address is correct (otherwise, correct the MAC address as it has to match what was entered in the synoboot.
8. Under "Controller USB 2", change to to "USB 3"
You are done and can now click on "Begin Installation". I normally just the the installation program download the latest version directly from Synology during the installation.