virtio_ring: Maintain a shadow copy of descriptors

virtio_ring: Maintain a shadow copy of descriptors

The shared descriptors should only be written by the guest driver,
however, the device is still able to overwrite and corrupt them.
Maintain a private shadow copy of the descriptors for the driver to
use for state tracking, removing the need to read from the shared
descriptors.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>