vmm_tests/petri: add ability to lower severity of com3 emitted logs, for tests that print too much to com3

Some tests (like the newly added MNF one) emit a lot of logs over com3 due to some components emitting a lot of logs. This is slow, and can cause timeout issues in the guest due to openhcl being too busy writing logs over emulated com3. This is especially apparent when openvmm is the virtstack, and running nested due to high intercept cost.

We need the ability to set the console serial threshold after we get a kmsg connection, so we can have the early logs available, but later logs only high severity ones go over com3. Petri should do this so that the tests are more reliable.

A design probably looks like adding a new explicit grpc message to change the com3 logging level that petri can send, once it has kmsg connected.