virtio: make virtqueue_alloc_element static

The function does not fully initialize the returned VirtQueueElement and should
be used only internally from the virtio module.

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