extra non-image storage added later breaks destroy action

Hi,
when you add further images to a uvt guest delete will still clean up fine in my experience.
But i you add something like a lun for example:

    <disk type='block' device='lun' rawio='no' sgio='filtered'>
      <driver name='qemu' type='raw'/>
      <source dev='/dev/disk/by-id/wwn-0x6005076306ffd6b60000000000002402'/>
      <target dev='sda' bus='scsi'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

Then the following uvt-kvm destroy will break with
  uvt-kvm: error: libvirt: key in virStorageVolLookupByKey must not be NULL

I assume we must harden the code a bit to skip over non file based disks (maybe with a info message).