eal: add device event monitor framework

eal: add device event monitor framework

This patch aims to add a general device event monitor framework at
EAL device layer, for device hotplug awareness and actions adopted
accordingly. It could also expand for all other types of device event
monitor, but not in this scope at the stage.

To get started, users firstly call below new added APIs to enable/disable
the device event monitor mechanism:
  - rte_dev_event_monitor_start
  - rte_dev_event_monitor_stop

Then users shell register or unregister callbacks through the new added
APIs. Callbacks can be some device specific, or for all devices.
  -rte_dev_event_callback_register
  -rte_dev_event_callback_unregister

Use hotplug case for example, when device hotplug insertion or hotplug
removal, we will get notified from kernel, then call user's callbacks
accordingly to handle it, such as detach or attach the device from the
bus, and could benefit further fail-safe or live-migration.

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