trace: allow trace events with string arguments

String arguments are useful for producing human-readable traces without
post-processing (e.g. stderr backend).  Although the simple backend
cannot handles strings all others can.  Strings should be allowed and
the simple backend can be extended to support them.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

docs/tracing.txt

@@ -70,11 +70,6 @@ Trace events should use types as follows:
    cannot include all user-defined struct declarations and it is therefore
    necessary to use void * for pointers to structs.
 
-   Pointers (including char *) cannot be dereferenced easily (or at all) in
-   some trace backends.  If pointers are used, ensure they are meaningful by
-   themselves and do not assume the data they point to will be traced.  Do
-   not pass in string arguments.
-
  * For everything else, use primitive scalar types (char, int, long) with the
    appropriate signedness.
 
@@ -182,6 +177,9 @@ source tree.  It may not be as powerful as platform-specific or third-party
 trace backends but it is portable.  This is the recommended trace backend
 unless you have specific needs for more advanced backends.
 
+The "simple" backend currently does not capture string arguments, it simply
+records the char* pointer value instead of the string that is pointed to.
+
 ==== Monitor commands ====
 
 * info trace