xnbd, xnbd_server: Convet setup device to setup session

Setup IO units list per session and divide each portal
thread with its share. Also move fstat logic to open
and only pack the file stat response in fstat command.

Each portal thread will contain a bs_dev instance of each
remote device over the session.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Fix Bug in stress IO

should make sure xio_msg->next is NULL.

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