xnbd: Rename blk_connection_data -> xnbd_connection

Also rename variables: conn_data -> xnbd_conn

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Rename session_data -> xnbd_session

Also rename variables blk_xnbd_session -> xnbd_session

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd.h: Align xnbd_connection struct

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Wait for all session connections establishment

when adding a new network-portal we must wait for
all connections to complete establishment before
allowing any other operations on the session.

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