[qemu] Port QEMU changes from `earlgrey_1.0.0` to `master`

OpenTitan QEMU support (execution environment, OTT updates, and related changes) was implemented on the `earlgrey_1.0.0` branch. We want to bring this functionality into master. Because many intermediate steps on the feature branch are no longer relevant to how we want to land the work, the port will not be done PR-by-PR; instead, we will perform a consolidated migration. This issue serves as the umbrella tracker for that porting effort and for any sub-tasks that arise during the process.

Start by comparing the OpenTitan QEMU support on the `earlgrey_1.0.0` branch with `master`, focusing on the execution environment, OTT updates, and related changes mentioned in the issue. Done means the required functionality has been consolidated on `master` and the resulting sub-tasks are tracked and resolved.

Written by the indexing model from the issue text.