If a storage volume's vhost_block_backend_version param is set, then a
Ubiblk installation is used instead of SPDK.
Let `storage-root=/var/storage/${vm_name}/$disk-index/`. Then:
- Ubiblk config is stored at `${storage-root}/vhost-backend.conf`.
- Ubiblk metadata is stored at `$storage-root/metadata`.
- A systemd-unit `${vm_name}-${disk-idx}-storage` is created to act as
VM's storage device. It starts the Ubiblk's vhost-backend binary,
stored at `/opt/vhost-block-backend/${version}/`.
- Key Encryption Key (KEK) is communicated to the storage service using
a pipe created at `${storage-root$}/kek.pipe`.
- Storage service creates a vhost-user-blk socket at
`${storage-root}/vhost.socket` which Cloud-Hypervisor connects to.
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