simpletrace: annotate magic constants from QEMU code

It wasn't clear where the constants and structs came from, so I added
comments to help.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230926103436.25700-3-mads@ynddal.dk
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/simpletrace.py

@@ -16,6 +16,11 @@
 
 __all__ = ['Analyzer', 'process', 'run']
 
+# This is the binary format that the QEMU "simple" trace backend
+# emits. There is no specification documentation because the format is
+# not guaranteed to be stable. Trace files must be parsed with the
+# same trace-events-all file and the same simpletrace.py file that
+# QEMU was built with.
 header_event_id = 0xffffffffffffffff
 header_magic    = 0xf2b177cb0aa429b4
 dropped_event_id = 0xfffffffffffffffe