@wolf.f -> Tricky Version or “Let’s make fun of Rockstor installer 
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Create your Virtio disks, follow the “serial number” post and create a “Fake Sata” disk (I’ve done a 2 GB disk): system will boot without errors (as it detects a sata disk) and you’ll be able to procede with installation, just remember to select destination and mark your Virtio HD instead of Sata HD 
Finally shutdown Rockstor, delete the Sata disk and restart VM.