net/virtio-user: support server mode

net/virtio-user: support server mode

In a container environment if the vhost-user backend restarts, there's
no way for it to reconnect to virtio-user. To address this, support for
server mode is added. In this mode the socket file is created by virtio-
user, which the backend then connects to. This means that if the backend
restarts, it can reconnect to virtio-user and continue communications.

With current implementation, LSC is enabled at virtio-user side to
support to accept the coming connection.

Server mode virtio-user only supports to work with vhost-user.

Release note is updated in this patch.

Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>