xnbd_server: Introduce xnbd_server

Up until now xnbd worked against raio_server. There
is a need to branch from raio_server since its support
in raio_client is too restricting. This server will
be adjusted to xnbd needs.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
xnbd_server: update .gitignore after adding xnbd_server

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>