Description of problem:
VMExport manifests were created for PVC source, but shouldn't have

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

How reproducible:
Always

Steps to Reproduce:
1. Create a DV
2. Create a VMExport for PVC
3. See the VMExport object has manifests
$ oc get vmexport export-pvc -o jsonpath=

{.status.links.external.manifests}

[

{"type":"all","url":"https://virt-exportproxy-openshift-cnv.apps.sno02-cnvqe2-rdu2.cnvqe2.lab.eng.rdu2.redhat.com/api/export.kubevirt.io/v1alpha1/namespaces/default/virtualmachineexports/export-pvc/external/manifests/all"}

,

{"type":"auth-header-secret","url":"https://virt-exportproxy-openshift-cnv.apps.sno02-cnvqe2-rdu2.cnvqe2.lab.eng.rdu2.redhat.com/api/export.kubevirt.io/v1alpha1/namespaces/default/virtualmachineexports/export-pvc/external/manifests/secret"}

]

Actual results:
VMExport object has manifests

Expected results:
VMExport from PVC source should not contain manifests

Additional info:

$ cat dv-lvm.yaml
apiVersion: cdi.kubevirt.io/v1beta1
kind: DataVolume
metadata:
name: dv-source
annotations:
cdi.kubevirt.io/storage.bind.immediate.requested: 'true'
spec:
source:
http:
url: "<cirros_url>"
storage:
resources:
requests:
storage: 1Gi
storageClassName: lvms-vg1
volumeMode: Block

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