tools lib traceevent: Use asprintf when possible

It makes the code clearer and less error prone.

clearer:
- less code
- the code is now using the same format to create strings dynamically

less error prone:
- no magic number +2 +9 +5 to compute the size
- no copy&paste of the strings to compute the size and to concatenate

The function `asprintf` is not POSIX standard but the program
was already using it. Later it can be decided to use only POSIX
functions, then we can easly replace all the `asprintf(3)` with a local
implementation of that function.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Link: http://lkml.kernel.org/r/20170802221558.9684-2-federico.vaga@vaga.pv.it
Link: http://lkml.kernel.org/r/20180112004822.686281649@goodmis.org


Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>