net: Add save_acked_features callback to vhost_net

net: Add save_acked_features callback to vhost_net

This commit introduces a save_acked_features function pointer to
vhost_net and converts the vhost_net function into a generic dispatcher.

The vhost-user backend provides the callback, making its function static.
With this change, no other module has a direct dependency on the
vhost-user implementation.

This cleanup allows for the complete removal of the net/vhost-user.h
header file.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>