net/virtio-user: support kernel vhost

This patch add support vhost kernel as the backend for virtio_user.
Three main hook functions are added:
  - vhost_kernel_setup() to open char device, each vq pair needs one
    vhostfd;
  - vhost_kernel_ioctl() to communicate control messages with vhost
    kernel module;
  - vhost_kernel_enable_queue_pair() to open tap device and set it
    as the backend of corresonding vhost fd (that is to say, vq pair).

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>