qemu-img: add special exit code if bdrv_check is not supported

currently it is not possible to distinguish by exitcode if there
has been an error or if bdrv_check is not supported by the image
format. Change the exitcode from 1 to 63 for the latter case.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -607,7 +607,7 @@ static int img_check(int argc, char **argv)
         if (output_format == OFORMAT_HUMAN) {
             error_report("This image format does not support checks");
         }
-        ret = 1;
+        ret = 63;
         goto fail;
     }