eal: add pending interrupt callback unregister

eal: add pending interrupt callback unregister

use case: if callback is used to receive message form socket,
and the message received is disconnect/error, this callback needs
to be unregistered, but cannot because it is still active.

With this patch it is possible to mark the callback to be
unregistered once the interrupt process is done with this
interrupt source.

Signed-off-by: Jakub Grajciar <jgrajcia@cisco.com>