QEMU: Support feature toggle for GA via serial port

[arixmkii]

Description

Follow up to #3148 (comment) Serial port mode was quick disabled by commenting out one line due to some instabilities, but other parts related kept untouched. First of all both parts should honor the same mode and as the supporting code is already there it could be extended to have feature toggle, so, the mode could be re-enabled if desired. This would also be vital for Windows hosts until Unix socket forwarding is figured out.

I plan to work on this - continuing and repurposing my draft PR #3148

[nirs]

The description #3184 could be useful here. It is hard to understand the problem from this issue description.

It is also important to understand if we really need to toggle this, and how do you plan to toggle it? A new lima.yaml configuration? extending existing configuration?

[arixmkii]

I was considering per machine setting inside lima.yaml in the QEMU related section, but I haven't checked the actual code yet and can't comment on a final decision yet.