vhost/crypto: fix feature negotiation

vhost/crypto: fix feature negotiation

This patch fixes the feature negotiation for vhost crypto during
initialization. The patch uses the newly created driver start
function to inform the driver type with the fixed vhost features.
In addition the patch provides a new API specifically used by
the application to start a vhost-crypto driver.

Fixes: 939066d96563 ("vhost/crypto: add public function implementation")
Cc: stable@dpdk.org
Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>