Remove unnecessary use of enable/disable callback notifications
and the incorrect more space available check.

The virtio_transport_tx_work handles when the TX virtqueue
has more buffers available.
Signed-off-by: Gerard Garcia <ggarcia@deic.uab.cat>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>