[2152122] VM can't start if disk io is default

Description of problem:
vm stuck in starting status if set disk io=default

Version-Release number of selected component (if applicable):
4.12

How reproducible:
always

Steps to Reproduce:
1. set vm disk io to default in vm yaml file
...
disks:

• disk:
  bus: virtio
  io: default
  name: rootdisk
  ...
  If set io to meaningless value, will get msg :

1. * spec.template.spec.domain.devices.disks.domain.devices.disks[0].io: Disk IO mode for spec.template.spec.domain.devices.disks.domain.devices.disks[0].io is not supported. Supported modes are: native, threads, default.

2. start vm
3.

Actual results:
$ oc get vm
NAME AGE STATUS READY
vm-fedora 48m Starting False

It will stuck, vm can't running

get error msg:
Warning SyncFailed 2s (x11 over 10s) virt-handler server error. command SyncVMI failed: "LibvirtError(Code=27, Domain=5, Message='XML error: Invalid value for attribute 'io' in element 'driver': 'default'.')"

Expected results:
If cnv support io default, vm should be started. if not support, should give right error msg.

Additional info: