net/virtio-user: abstract backend operations

net/virtio-user: abstract backend operations

Add a struct virtio_user_backend_ops to abstract three kinds of backend
operations:
  - setup, create the unix socket connection;
  - send_request, sync messages with backend;
  - enable_qp, enable some queue pair.

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