Merge remote-tracking branch 'stefanha/tracing' into staging

* stefanha/tracing:
  vga: add trace event for ppm_save
  console: add some trace events
  maintainers: Add docs/tracing.txt to Tracing
  docs: correct ./configure line in tracing.txt
  trace: make trace_thread_create() use its function arg
  tracetool: Omit useless QEMU_*_ENABLED() check
  trace: Provide a per-event status define for conditional compilation