lisa.trace: Restrict trace_printk@myevent supported events

In order to support trace_printk meta events more easily in upcoming
parsers, restrict the format that is accepted so that:

* the format string is only used to infer the format of the event
* the variable arguments buffer is used for the values
* we must get a bprint, not bputs event

lisa/trace.py

@@ -4010,23 +4010,31 @@ def df_event(self, event, raw=None, window=None, signals=None, signals_init=True
             In addition to actual events, the following formats for meta events
             are supported:
 
-            * ``trace_printk@``: the event will be assumed to be embedded in
-              textual form inside the field of another event as a string,
-              typically emitted using the ``trace_printk()`` kernel function:
+            * ``trace_printk@``: The event format is described by the
+              ``bprint`` event format string, and the field values are decoded
+              from the variable arguments buffer. Note that:
+
+                * The field values *must* be in the buffer, i.e. the format
+                  string is only used as the event format, no "literal value"
+                  will be extracted from it.
+
+                * The event *must* have fields. If not, ``trace_printk()``
+                  will emit a bputs event that will be ignored at the moment.
+                  We need to get a bprint event.
 
               .. code-block:: C
 
-                  // trace.df_event('trace_printk@event')
+                  // trace.df_event('trace_printk@myevent')
                   void foo(void) {
-                      trace_printk("event: field1=foo field2=42");
+                      trace_printk("myevent: field1=%s field2=%i", "foo", 42);
                   }
 
             * ``userspace@``: the event is generated by userspace:
 
               .. code-block:: shell
 
-                  # trace.df_event('userspace@event')
-                  echo "event: field1=foo field2=42" > /sys/kernel/debug/tracing/trace_marker
+                  # trace.df_event('userspace@myevent')
+                  echo "myevent: field1=foo field2=42" > /sys/kernel/debug/tracing/trace_marker
 
             .. note:: All meta event names are expected to be valid C language
                 identifiers. Usage of other characters will prevent correct