gpudev: add event notification

gpudev: add event notification

Callback functions may be registered for a device event.
Callback management is per-process and not thread-safe.

The events RTE_GPU_EVENT_NEW and RTE_GPU_EVENT_DEL
are notified respectively after creation and before removal
of a device, as part of the library functions.
Some future events may be emitted from drivers.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>