virstoragetest: Add case for qcow2 image with empty string as 'data_file'

virstoragetest: Add case for qcow2 image with empty string as 'data_file'

Add an example image formatted by:

 qemu-img create -f qcow2 -o data_file=nbd+unix:///datafile?socket=/tmp/nbd,data_file_raw=true /tmp/nbddatastore.qcow2 10M -u

serving as an example when qemu records an empty string as the
'data_file' field.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>