app/testpmd: add generic event handler
app/testpmd: add generic event handler

This is a rather simple handler that prints a message with the name of
the current event. It can be used to check PMD callback registration and
triggers.

Signed-off-by: Gaetan Rivet <a6b9f1aecd7707f01344f211143eb3bcaeb0c5c3@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk