tracing: Fix polling on trace_pipe_raw

The trace_pipe_raw never implemented polling and this was casing
issues for several utilities. This is now implemented.

Blocked reads still are on the TODO list.

Reported-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>