Hi,
The "manual migration" option is only a default value. You can change the migration mode to "automatic" mode if you prefer.
Details:
When you enable Host CPU Pass-Through for a VM then the migration default value is auto set to "manual migration" in UI because it's not sure if this VM should be automatically migrated as part of internal events (such as load balancing policies or host turning into maintenance mode), without letting the user controlling it.
Sometimes there are few destination hosts to migrate to and we want to let the user controlling that since performance may decrease or migration may fail.
But if you are sure that the cluster contains hosts with identical cpu model and identical number of sockets, threads and cores as the source host, and you don't mind that VM will be migrated whenever required, then you can definitely change the migration value to "Allow manual and automatic migration".