vhost: add external message handling to the API

vhost: add external message handling to the API

External message callbacks are used e.g. by vhost crypto
to parse crypto-specific vhost-user messages.

We are now publishing the API to register those callbacks,
so that other backends outside of DPDK can use them as well.

Signed-off-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>