This patch introduces new VhostOps vhost_set_config_call. This function allows the
vhost to set the event fd to kernel

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20211104164827.21911-5-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>