migration-test: Use migrate_ensure_converge() for auto-converge

Thomas reported that auto-converge test will timeout on MacOS CI gatings.
Use the migrate_ensure_converge() helper too in the auto-converge as when
Daniel reworked the other test cases.

Since both max_bandwidth / downtime_limit will not be used for converge
calculations, make it simple by removing the remaining check, then we can
completely remove both variables altogether, since migrate_ensure_converge
is used the remaining check won't make much sense anyway.

Reported-by: Thomas Huth <thuth@redhat.com>
Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220728133516.92061-2-peterx@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>