eal: modify device event process function

This patch modify the device event callback process function name to be
more explicit, change the variable to be const. And more, because not only
eal device helper will use the callback, but also vfio bus will use the
callback to handle hot-unplug, so exposure the API out from private eal.
The bus drivers and eal device would directly use this API to process
device event callback.

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>