doc: add vhost-user guide
doc: add vhost-user guide

Add vhost user documentation

Signed-off-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>
Acked-by: Siobhan Butler <8cf2da75f16c15b95e66f52a35c85d9ba5dc91d6@intel.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk