tests/migration: Add some slack to auto converge

There's an assert in autoconverge that checks that we quit the
iteration when we go below the expected threshold.  Philippe
saw a case where this assert fired with the measured value
slightly over the threshold. (about 3k out of a few million).

I can think of two reasons:
  a) Rounding errors
  b) That after we make the decision to quit iteration we do one
    more sync and that sees a few more dirty pages.

So add 1% slack to the assertion, that should cover a and
most cases of b, probably all we'll see for the test.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>

tests/qtest/migration-test.c

@@ -1237,7 +1237,8 @@ static void test_migrate_auto_converge(void)
     g_assert_cmpint(percentage, <=, max_pct);
 
     remaining = read_ram_property_int(from, "remaining");
-    g_assert_cmpint(remaining, <, expected_threshold);
+    g_assert_cmpint(remaining, <,
+                    (expected_threshold + expected_threshold / 100));
 
     migrate_continue(from, "pre-switchover");