fdstream: Emit stream abort callback even if poll() doesnt.

fdstream: Emit stream abort callback even if poll() doesnt.

This patch causes the fdstream driver to call the stream event callback
if virStreamAbort() is called on a stream using this driver.

A remote handler for a stream can only detect changes via stream events,
so this event callback is necessary in order to enable a daemon to abort
a stream in such a way that the client will see the change.

* src/fdstream.c:
        - modify close function to call stream event callback