Description of problem:
When starting a VM with preference with the PreferredStorageClassName field, we must specify dataVolumeTemplates[0].spec.pvc, this will also auto populate dataVolumeTemplates[0].spec.pvc.resource which conflict with dataVolumeTemplates[0].spec.storage.resources.requests.storage

Version-Release number of selected component (if applicable):
4.14.0

How reproducible:
100%

Steps to Reproduce:
1. Create VM with preference with PreferredStorageClassName
2. Add dataVolumeTemplates[0].spec.pvc.name to the VM spec
3. 

Actual results:
The VMI fail to be created with conflict error of 2 resources fields defined

Expected results:
The VMI should be created

Additional info:
an example of the conflict fields: 
    dataVolumeTemplates:
    - apiVersion: cdi.kubevirt.io/v1beta1
      kind: DataVolume
      metadata:
        creationTimestamp: null
        name: fedora
      spec:
        pvc:
          resources: {}
          storageClassName: nfs
          volumeName: volume-for-test
        sourceRef:
          kind: DataSource
          name: fedora
          namespace: openshift-virtualization-os-images
        storage:
          resources:
            requests:
              storage: "34087042032"