tracetool: clarify that "formats" means "format strings"

The terminology used by tracetool is not consistent with C sprintf or
docs/devel/tracing.txt.  The word "formats" is sometimes used to mean
"format strings".

This patch clarifies comments and error messages that contain this word.

Note that the error message lines are longer than 80 characters but I
have not wrapped them to aid grepping.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180110202553.31889-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/tracetool/__init__.py

@@ -173,7 +173,7 @@ class Event(object):
         props : list of str
             Property names.
         fmt : str, list of str
-            Event printing format (or formats).
+            Event printing format string(s).
         args : Arguments
             Event arguments.
         orig : Event or None
@@ -237,9 +237,9 @@ class Event(object):
         if "tcg-exec" in props:
             raise ValueError("Invalid property 'tcg-exec'")
         if "tcg" not in props and not isinstance(fmt, str):
-            raise ValueError("Only events with 'tcg' property can have two formats")
+            raise ValueError("Only events with 'tcg' property can have two format strings")
         if "tcg" in props and isinstance(fmt, str):
-            raise ValueError("Events with 'tcg' property must have two formats")
+            raise ValueError("Events with 'tcg' property must have two format strings")
 
         event = Event(name, props, fmt, args)
 
@@ -263,7 +263,7 @@ class Event(object):
     _FMT = re.compile("(%[\d\.]*\w+|%.*PRI\S+)")
 
     def formats(self):
-        """List of argument print formats."""
+        """List conversion specifiers in the argument print format string."""
         assert not isinstance(self.fmt, list)
         return self._FMT.findall(self.fmt)