trace: Remove %s in grlib trace events

Trace events cannot use %s in their format strings because trace
backends vary in how they can deference pointers (if at all).  Recording
const char * values is not meaningful if their contents are not recorded
too.

Change grlib trace events that rely on strings so that they communicate
similar information without using strings.

A follow-up patch explains this limitation and updates docs/tracing.txt.

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