qemudriver: Allow machine-specific options and pre-start QMP commands

[JSydll]

Description

Two minor features are added to the QemuDriver:

Support machine-specific options (e.g. -machine virt,secure=on)

While raising an exception for unsupported machines/boards is reasonable, adding machine-specific options shall not break the parsing. Therefore, the check is adjusted to only account for the leading board name.

Support interactions with the QEMU instance via QMP before releasing CPU(s)

Especially for bad case testing, it can be useful to be able to interact with the prepared but not yet spun off instance - e.g. to corrupt the boot medium without the need to build a corrupted image only for that purpose.

For both features, straight forward unit tests are provided.
Lastly, the changeset includes minor documentation updates and the introduction of type hints for the qemudriver.py file.

Checklist

• Documentation for the feature
• Tests for the feature
• PR has been tested

[Emantor]

Rather than adding a pre-start hook, I think it is more clear to expose somthing like a prepare() function which sets up the qemu process, and than have on() only do the cont monitor command. For backwards compatibility, the class can check whether the self._child attribute exists and call prepare() in on() if that was not done yet.

[Emantor]

I think we can take the other fixes in this PR as well.

[JSydll]

Thanks for the review and sorry for force-pushing so often - it looked easier to me to fix the one linting issue I overlooked when implementing the proposal from Rouven.

[Emantor]

It's all good and thanks for keeping up with the PR even if it takes some time on our side 👍

[JSydll]

I shortly glimpsed into the failing pipeline and now I'm wondering whether some environment changes happened? The test_qemudriver is not able to locate qemu-system-arm...