Allow specifying guest trace level for different modules

[dblnz]

We currently have a single trace level for everything inside the guest (all the modules).
We currently have no way to specify a trace level for the guest but a different one for the host (for the same module).
I'll make a separate issue to tackle this.

This is possible to imply a change of API for the entrypoint.

Moreover, we should think of a way to do this per guest call, because going forward, taking a snapshot after creation means the trace level cannot be changed afterwards.

[simongdavies]

Linked to #1268