qemu-img: fix img_commit() error return value

The img_commit() return value is a process exit code.  Use 1 for failure
instead of -1.  The other failure paths in this function already use 1.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

qemu-img.c

@@ -752,7 +752,7 @@ static int img_commit(int argc, char **argv)
     ret = bdrv_parse_cache_flags(cache, &flags);
     if (ret < 0) {
         error_report("Invalid cache option: %s", cache);
-        return -1;
+        return 1;
     }
 
     bs = bdrv_new_open("image", filename, fmt, flags, true, quiet);