qemu-*: move from testing to main

Note: Do not merge this after two approvals. See https://wiki.postmarketos.org/wiki/Device_categorization#Moving_to_a_higher_category (discuss in https://gitlab.com/postmarketOS/postmarketos/-/issues/24 and https://gitlab.com/postmarketOS/postmarketos/-/issues/25).

For testing changes for device categorization, it is useful to have a device in each of the categories. The PinePhone is close to being moved to main/, but it doesn't fulfill all requirements yet.

The QEMU "device ports" are very simple since QEMU currently only emulates a rather limited set of hardware features. All available features are working correctly (especially after the recent rework of the QEMU packages). I suppose it is also usable as a "daily driver", at least for its intended purpose (a virtual machine for testing postmarketOS changes). :)

Given that everyone can run QEMU, everyone could potentially maintain it. For now I have added myself as maintainer since I did most of the recent cleanup.

Overall it seems useful to have qemu-* in main/, especially because it is now the device that is selected by default in pmbootstrap.
I can try to come up with more reasons to have it in main/, if anyone needs more to be convinced. :)

EDIT by ollieparanoid: link to review process in wiki