client: Report proper close reason
    
    When we get a POLLHUP or VIR_EVENT_HANDLE_HANGUP event for a client, we
    still want to read from the socket to process any accumulated data. But
    doing so inevitably results in an error and a call to
    virNetClientMarkClose before we get to processing the hangup event (and
    another call to virNetClientMarkClose). However the close reason passed
    to the second virNetClientMarkClose call is ignored because another one
    was already set. We need to pass the correct close reason when marking
    the socket to be closed for the first time.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1373859
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>