virtio-fs: add fuse protocol header

This file is a copy of https://github.com/libfuse/libfuse/blob/38c9cb43787bf83ca4e9c9af707e82165de99008/include/fuse_kernel.h
and provides the wire-level interface of fuse protocol used to exchange
data between guest and host over virtio-fs device.

Signed-off-by: Waldemar Kozaczuk <jwkozaczuk@gmail.com>
Message-Id: <20200208232200.15009-1-jwkozaczuk@gmail.com>

Author: wkozaczuk