ioeventfd: error handling cleanup - Don't return status from start/stop functions where it's ignored - report errors to make debugging easier - assert on unexpected failures - don't disable notifiers on error so that we'll retry when guest driver restarts Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>