virtio-serial-bus: Ports are expected to implement 'have_data' callback

There's no need to check if ports can accept any incoming data from the
guest each time the guest sends data.  Check if the port implements such
functionality during port initialisation.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
