lib: generate trace events for log messages

lib: generate trace events for log messages

LTTng supports tracef() and tracelog() macros, which work like printf,
and are used to ease transition between logging and tracing. Messages
printed using these macros end up as trace events. For our uses we are
not interested in dropping logging, but it is nice to get log messages
in trace output, so I've added a call to tracelog() in zlog that dumps
our zlog messages as trace events.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>