iotests: fix 194: filter out racy postcopy-active event

iotests: fix 194: filter out racy postcopy-active event

The event is racy: it will not appear in the output if bitmap is
migrated during downtime period of migration and postcopy phase is not
started.

Fixes: ae00aa239847 "iotests: 194: test also migration of dirty bitmap"
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230607143606.1557395-1-vsementsov@yandex-team.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>