vhost: rework message handling as a callback array

vhost: rework message handling as a callback array

Introduce vhost_message_handlers, which maps the message request
type to the message handler. Then replace the switch construct
with a map and call.

Failing vhost_user_set_features is fatal and all processing should
stop immediately and propagate the error to the upper layers. Change
the code accordingly to reflect that.

Signed-off-by: Nikolay Nikolaev <nicknickolaev@gmail.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>