Difference between virtio and virtio single controller types?

[gkovacs]

the VM configuration is checked at the beginning of the backup and an error message is returned if iothreads are enabled on any of the disks that should be included in the backup.

Right, so I disabled the iothread option on the individual disks, yet the controller type is still set to virtio single, and the guest Windows still shows 3 controllers connected to the 3 disks. Backup completed successfully. Does this config mean I'm still getting one IO thread for each disk, or in this case all 3 controllers use the master thread?