Fix leak of virStreamPtr object with callback added in fdstream impl

Fix leak of virStreamPtr object with callback added in fdstream impl

When adding a callback to an FD stream, we take an extra reference
on the virStreamPtr instance. We forgot to registered a free function
with the callback, so when the callback was removed, the extra
reference held on virStreamPtr was not released.

* src/fdstream.c: Use a free callback to release reference on
  virStreamPtr when removing callback