Allow for configuration overrides in VM config using metadata

[SapphicCode]

This is good if, for instance, you want a build host or a game server (which allocate memory quite fast) to have enough headroom to allocate memory.

We should be able to store these config overrides in the domain metadata and cache them into balloond when it's making use of them.

Another useful addition would be the ability to manually disable management of a VM's balloon driver without removing the balloon driver itself from the VM.