virtio-serial: make flow control explicit in virtio-console

virtio-serial: make flow control explicit in virtio-console

virtio-console.c used to return a value less than the number of bytes
asked to be written out to a chardev backend in case the backend is not
writable.  virtio-serial-bus.c then implicitly enabled flow control for
that port.

Make this explicit instead.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Message-id: f5ec50b068c25422256e499cf4adc06d353bf394.1362505276.git.amit.shah@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>