trace: allow trace events with string arguments

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>