vhost: Do not depend on !NULL VirtQueueElement on vhost_svq_flush
Since QEMU will be able to inject new elements on CVQ to restore the
state, we need not to depend on a VirtQueueElement to know if a new
element has been used by the device or not. Instead of check that, check
if there are new elements only using used idx on vhost_svq_flush.
Signed-off-by: Eugenio Pérez <eperezma@redhat.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Jason Wang <jasowang@redhat.com>