I managed to get to the Synology Assistant by setting the disk into VirtIO mode. But then no disk are found. If I change it back to AHCI everything stays black after grub. Any suggestions?
I've tried the steps of @PettyGenius but it can't get it to work. If I restart the VM en go to console quickly I can see the bootloader GRUB menu, but after selecting one of the items the screen goes black, nothing happens.