virstoragetest: Format detected/unprocessed backing store format into output files

virstoragetest: Format detected/unprocessed backing store format into output files

Compare also the detected format of the backing file
('backingStoreRawFormat' field) into the output data for comparison with
others. Since the ToString function can't convert VIR_STORAGE_FILE_AUTO
use also the numeric value.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>