iotests: Fix "no qualified output" error path

The variable for error messages to be displayed is $results, not
$reason. Fix 'check' to print the "no qualified output" error message
again instead of having a failure without any message telling the user
why it failed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/check

@@ -887,7 +887,7 @@ do
             if [ ! -f "$reference" ]
             then
                 status="fail"
-                reason="no qualified output"
+                results="no qualified output"
                 err=true
             else
                 if diff -w "$reference" $tmp.out >/dev/null 2>&1