petri: use com3 for kmsg logs

[tjones60]

Using COM3 instead of diag_client allows us to get OpenHCL logs if it crashes early in boot

[tjones60]

I tested this and although it configured the serial port successfully, it didn't seem to actually log any data from COM3 (I was able to connect to COM3 via hypestv while the test was running, which should not be possible, haha).

So it seems like something is missing.

Yeah I've been focusing on other things, but I did notice that the logs aren't actually coming through reliably (if you download the logs from the tdx/arm test runs on this PR which use com3, you can see that it sometimes gets some of the logs). I hope to get the time to investigate more soon.

[github-actions[bot]]

At least one Petri test failed.

[benhillis]

Backported to release/2505 in #1947