iotests/270: Don't store data-file with json: prefix in image

iotests/270: Don't store data-file with json: prefix in image

We want to disable filename parsing for data files because it's too easy
to abuse in malicious image files. Make the test ready for the change by
passing the data file explicitly in command line options.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
(cherry picked from commit 7e1110664ecbc4826f3c978ccb06b6c1bce823e6)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>