I can share some test results with you:
1. If no memtune->hard_limit is set when start a vm, the default memlock hard limit is 64MB
2. If memtune->hard_limit is set when start a vm, memlock hard limit will be set to the value of memtune->hard_limit
3. If memtune->hard_limit is updated at run-time, memlock hard limit won't be changed accordingly
And some additional knowledge:
1. memlock hard limit can be shown by ‘prlimit -p <pid-of-qemu> -l’
2. The default value of memlock hard limit can be changed by setting LimitMEMLOCK in /usr/lib/systemd/system/virtqemud.service