Description of problem:
Cant preallocate blank block DV

Version-Release number of selected component (if applicable):
CNV 2.6.0

How reproducible:
100%

Steps to Reproduce:
1. Create a blank block DV with preallocated: true

Actual results:
cdi.kubevirt.io/storage.preallocation is not present on PVC

Expected results:
cdi.kubevirt.io/storage.preallocation annotation on PVC should be "true"(preallocation occurred)

Additional info:
Comment #1

[cnv-qe-jenkins@stg01-alex-rjlb7-executor prealloc-block-volmode-bugs]$ cat blank_block_dv.yaml 
---
apiVersion: cdi.kubevirt.io/v1beta1
kind: DataVolume
metadata:
  name: cnv-5737
spec:
  source:
    blank: {}
  pvc:
    accessModes:
    - ReadWriteMany
    resources:
      requests:
        storage: 100Mi
    storageClassName: ocs-storagecluster-ceph-rbd
    volumeMode: Block
  preallocation: true