Description of the problem:
VMExport: can't download a PVC that was created from DV on NFS (when there's no VM that owns this DV/PVC)

Version-Release number of selected component (if applicable):
4.12, 4.13

How reproducible:
Always on NFS, when DV/PVC is not owned by a VM
It works with NFS when VM owns the DV/PVC
It works with HPP and OCS with no regard to a VM

Steps to Reproduce:
1. Create an NFS DV
2. Create a VMExport for PVC
3. See the VMExport is Ready
$ oc get vmexport
NAME SOURCEKIND SOURCENAME PHASE
export-pvc-nfs PersistentVolumeClaim dv-source Ready

4. Try to download the image, but get an error
$ virtctl vmexport download export-pvc-nfs --output=disk-pvc-nfs.img --keep-vme
Processing completed successfully
Bad status: 500 Internal Server Error

5. See the log:

$ oc logs virt-export-export-pvc-nfs | grep error

{"component":"virt-exportserver-virt-export-export-pvc-nfs","level":"error","msg":"error opening /export-volumes/dv-source/disk.img","pos":"exportserver.go:315","reason":"open /export-volumes/dv-source/disk.img: permission denied","timestamp":"2022-12-27T09:22:32.371279Z"}

Actual results:
Bad status: 500 Internal Server Error

Expected results:
Image downloaded successfully

Additional info:

$ cat dv-nfs.yaml

apiVersion: cdi.kubevirt.io/v1alpha1
kind: DataVolume
metadata:
name: dv-source
spec:
source:
http:
url: <cirros.img>
storage:
resources:
requests:
storage: 1Gi
storageClassName: nfs
volumeMode: Filesystem

$ cat vmexport-dv-nfs.yaml

apiVersion: export.kubevirt.io/v1alpha1
kind: VirtualMachineExport
metadata:
name: export-pvc-nfs
spec:
source:
apiGroup: ""
kind: PersistentVolumeClaim
name: dv-source