tests/migration: Add some slack to auto converge

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]>