virtio-serial: add missing virtio_detach_element() call

Ports enter a "throttled" state when writing to the chardev would block.
The current output VirtQueueElement is kept around until the chardev
becomes writable again.

There are several places in the virtio-serial lifecycle where the
VirtQueueElement should be thrown away.  For example, if the virtio
device is reset then virtqueue elements are no longer valid.

This patch adds the discard_throttle_data() function to unmap the
scatter-gather list and decrement vq->inuse.  This ensures that the
VirtQueueElement is freed properly.

Cc: amit.shah@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
