app/testpmd: enable device hotplug monitoring

app/testpmd: enable device hotplug monitoring

Use testpmd for example, to show how an application uses device event
APIs to monitor the hotplug events, including both hot removal event
and hot insertion event.

The process is that, testpmd first enable hotplug by below commands,

E.g. ./build/app/testpmd -c 0x3 --n 4 -- -i --hot-plug

then testpmd starts the device event monitor by calling the new API
(rte_dev_event_monitor_start) and register the user's callback by call
the API (rte_dev_event_callback_register), when device being hotplug
insertion or hotplug removal, the device event monitor detects the event
and call user's callbacks, user could process the event in the callback
accordingly.

This patch only shows the event monitoring, device attach/detach would
not be involved here, will add from other hotplug patch set.

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>