tests/migration: Tweek auto converge limits check

tests/migration: Tweek auto converge limits check

Thomas found an autoconverge test failure where the
migration completed before the autoconverge had kicked in.

To try and avoid this again:
  a) Reduce the usleep in test_migrate_auto_converge
    so that it should exit quicker when autoconverge kicks in
  b) Make the loop exit immediately rather than have the sleep
     when it does start autoconverge, otherwise the autoconverge
     might succeed during the sleep.
  c) Reduce inc_pct so auto converge happens more slowly
  d) Reduce the max-bandwidth in migrate_ensure_non_converge
    to make the ensure more ensure.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20230306152612.52291-1-dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>