[OpenBIOS] Grand Unified Tracing

[Blue Swirl]

OpenBIOS uses traditional DPRINTFs for debugging. I was considering
replacing those with tracepoints, which would output to serial device
or whatever DPRINTFs are using currently. This would not be extremely
useful by itself, except maybe that configuration for debugging would
be concentrated to single 'trace-events' file, but this would not be a
major improvement over current XML configuration.

But developing this further, maybe OpenBIOS could also pass the
tracepoint data back to QEMU? Then all tracepoint data would be
synchronized, coherent and all gathered to single place.

The implementation could be that fw_cfg would be used to pass
simpletrace style data. An offset should be added to event IDs and
data would then be output as usual. On OpenBIOS side, the
implementation would be pretty similar to current QEMU tracepoints but
in place of file output there would be fw_cfg output.

Syntax for trace-events file should be augmented with include
directive, so that QEMU knows also OpenBIOS tracepoints. I think the
only change to simpletrace.py would be to parse this directive.

Controlling OpenBIOS tracepoints from QEMU monitor would be cool too.

Going even further, other targets like kernels could use something
similar, probably not using fw_cfg though.

What do you think?