net/virtio-user: add vhost-user adapter layer

net/virtio-user: add vhost-user adapter layer

This patch provides vhost adapter layer implementation. Two main
help functions are provided to upper layer (device emulation):
  - vhost_user_setup(), to set up vhost user backend;
  - vhost_user_sock(), to talk with vhost user backend.

  ----------------------
  | ------------------ |
  | | virtio driver  | |
  | ------------------ |
  |         |          |
  | ------------------ | ------>  virtio-user PMD
  | | device emulate | |
  | |                | |
  | | vhost adapter  |-|----> (vhost_user.c)
  | ------------------ |
  ----------------------
            |
            | -------------- --> (vhost-user protocol)
            |
   ------------------
   | vhost backend  |
   ------------------

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>