block/aio_task: assert `max_busy_tasks` is greater than 0

All code in block/aio_task.c expects `max_busy_tasks` to always
be greater than 0.

Assert this condition during the AioTaskPool creation where
`max_busy_tasks` is set.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211005161157.282396-3-sgarzare@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>