iotests/197: Fix for non-qcow2 formats

iotests/197: Fix for non-qcow2 formats

While 197 is very much a qcow2 test, and it looks like the partial
cluster case at the end (introduced in b0ddcbbb36a66a6) is specifically
a qcow2 case, the whole test scripts actually marks itself to work with
generic formats (and generic protocols, even).

Said partial cluster case happened to work with non-qcow2 formats as
well (mostly by accident), but 1855536256 broke that, because it sets
the compat option, which does not work for non-qcow2 formats.

So go the whole way and force IMGFMT=qcow2 and IMGPROTO=file, as done in
other places in this test.

Fixes: 1855536256eb0a5708b04b85f744de69559ea323
       ("iotests/197: Fix for compat=0.10")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200728131134.902519-1-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>