tools lib traceevent: Report unknown VMX exit reasons with code

Allows to parse the result even if the KVM plugin does not yet
understand a specific exit code.

Link: http://lkml.kernel.org/r/5207446F.1090703@web.de
Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>