docs/nvdimm: add example on persistent backend setup

docs/nvdimm: add example on persistent backend setup

Persistent backend setup requires some knowledge about nvdimm and ndctl
tool. Some users report they may struggle to gather these knowledge and
have difficulty to setup it properly.

Here we provide two examples for persistent backend and gives the link
to ndctl. By doing so, user could try it directly and do more
investigation on persistent backend setup with ndctl.

Signed-off-by: Wei Yang <[email protected]>
Reviewed-by: Pankaj Gupta <[email protected]>
Message-Id: <20190801004053[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>