Description of the problem:
When we create a VMExport object for VMSnapshot – volume names in VMExport object are export_name + volume_name, but we expect the volume names to be the same as the VMs volume names. This will let us use the 'virtctl vmexport download --volume=' command without looking at the vmexport's yaml. It's only an issue when VM has multiple volumes. When there's only one volume – it'll be successfully downloaded.
 
Version-Release number of selected component (if applicable):
4.12

How reproducible:
Always

Steps to Reproduce:
1. Create a VM with one DV
2. Create a VM's memory-dump (to have one more volume)
$ virtctl memory-dump get vm-cirros-source-ocs --claim-name=mem-dump-pvc --create-claim --storage-class=ocs-storagecluster-ceph-rbd
PVC default/mem-dump-pvc created
Successfully submitted memory dump request of VM vm-cirros-source-ocs

3. Create a VMSnapshot
4. Create a VMExport for the snapshot
$ virtctl vmexport create snap1-export --snapshot=my-vmsnapshot
VirtualMachineExport 'default/snap1-export' created succesfully

Please fix the typo in the "succesfully" word ^^

Actual results:
The name of the volume is export_name + volume_name
$ oc get vmexport snap1-export -oyaml
  ...
    volumes:      
         - name: snap1-export-cirros-dv-source-ocs
           ...
         - name: snap1-export-mem-dump-pvc

$ virtctl vmexport download snap1-export --volume=cirros-dv-source-ocs --output=disk.img --keep-vme
Processing completed successfully
Unable to get a valid URL from 'default/snap1-export' VirtualMachineExport

Expected results:
The name of the volume is the same as the VM's volume_name – cirros-dv-source-ocs,
virtctl vmexport download worked with --volume=cirros-dv-source-ocs

Additional info:

VMs yaml:

$ cat vm-ocs.yaml
apiVersion: kubevirt.io/v1alpha3
kind: VirtualMachine
metadata:
  name: vm-cirros-source-ocs
  labels:
    kubevirt.io/vm: vm-cirros-source-ocs
spec:
  dataVolumeTemplates:
  - metadata:
      name: cirros-dv-source-ocs
    spec:
      storage:
        resources:
          requests:
            storage: 1Gi
        storageClassName: ocs-storagecluster-ceph-rbd
      source:
        http:
          url: <cirros-0.4.0-x86_64-disk.qcow2>
  running: true
  template:
    metadata:
      labels:
        kubevirt.io/vm: vm-cirros-source-ocs
    spec:
      domain:
        devices:
          disks:
          - disk:
              bus: virtio
            name: datavolumev-ocs
        machine:
          type: ""
        resources:
          requests:
            memory: 100M
      terminationGracePeriodSeconds: 0
      volumes:
      - dataVolume:
          name: cirros-dv-source-ocs
        name: datavolumev-ocs

Snapshot yaml:

$ cat snapshot.yaml
apiVersion: snapshot.kubevirt.io/v1alpha1
kind: VirtualMachineSnapshot
metadata:
  name: my-vmsnapshot
spec:
  source:
    apiGroup: kubevirt.io
    kind: VirtualMachine
    name: vm-cirros-source-ocs