Hi @morgancox welcome on board!
Try to give some answers:
Virtio disks under KVM, try this Fake sata disk for Rockstor over Proxmox / KVM .
I had the same problem and the solution was to add x+1 disks (1 “fake sata”, 1 Virtio for Rockstor install, 2 Virtio for Btrfs): Rockstor on install detect a sata so doesn’t stop, you change installation device to your Virtio and, once installation is finished, shutdown VM, destroy sata disk and happy reboot 
(just remember the part of serial numbers!)
Backup on rockstor: you can use Rockstor as a nas via a samba share, taking snapshots to have files history, but snapshots don’t mean backup (backup is a copy of your data, btrfs+snapshots let you versioning your files)
Possible solution? Rockstor as central data storage (btrfs raid 1 + snapshots) + backup (rsync) to another local/remote disk
Flyer
P.S.: For every disk matter ask @phillxnet disk and smart troubles ninja 