vhost-user: keep vhost_net after a disconnection

vhost-user: keep vhost_net after a disconnection

Many code paths assume get_vhost_net() returns non-null.

Keep VhostUserState.vhost_net after a successful vhost_net_init(),
instead of freeing it in vhost_net_cleanup().

VhostUserState.vhost_net is thus freed before after being recreated or
on final vhost_user_cleanup() and there is no need to save the acked
features.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>