instance create saga should produce useful log messages #3877
Description
In recent investigations of instance creation times for large instances, I found that there were nearly nothing in the nexus logs around instance creation, and was reliant only on propolis-server logs for details there. This was problematic in cases where the instance failed to be created (similar failure mode to #3417), because there was no propolis zone for me to inspect.
We should beef up the instance creation saga logging with an eye toward debuggability here.