virtio-serial: Implement flow control for individual ports

virtio-serial: Implement flow control for individual ports

Individual ports can now signal to the virtio-serial core to stop
sending data if the ports cannot immediately handle new data.  When a
port later unthrottles, any data queued up in the virtqueue are sent to
the port.

Disable throttling once a port is closed (and we discard all the
unconsumed buffers in the vq).

The guest kernel can reclaim the buffers when it receives the port close
event or when a port is being removed. Ensure we free up the buffers
before we send out any events to the guest.

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