virNetSocketRemoveIOCallback: Be explicit about unref

virNetSocketRemoveIOCallback: Be explicit about unref

When going through the code I've notice that
virNetSocketAddIOCallback() increases the reference counter of
@socket. However, its counter part RemoveIOCallback does not. It took
me a while to realize this disproportion. The AddIOCallback registers
our own callback which eventually calls the desired callback and then
unref the @sock. Yeah, a bit complicated but it works. So, lets note
this hard learned fact in a comment in RemoveIOCallback().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>