vhost/crypto: add vhost-user message handlers

vhost/crypto: add vhost-user message handlers

Previously, vhost library lacks the support to the vhost backend
other than net such as adding private data or registering vhost-user
message handlers. This patch fills the gap by adding data pointer and
vhost-user pre and post message handlers to vhost library.

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Jay Zhou <jianjay.zhou@huawei.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>