virtqueue_discard() requires a VirtQueueElement but virtio-balloon does
not migrate its in-use element.  Introduce a new function that is
similar to virtqueue_discard() but doesn't require a VirtQueueElement.

This will allow virtio-balloon to access element again after migration
with the usual proviso that the guest may have modified the vring since
last time.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>