virtio-serial: Check if virtio queue is ready before consuming data

If a virtio-serial port is removed before the guest comes up and
initialises the virtqueues, qemu exits with the message

Guest moved used index from 0 to 61440

This happens because we try to clear any pending buffers from the
virtqueue.

Ensure the virtqueue is initialised before calling any virtqueue
operations.

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