xnbd: Use iou_pool per context

In order to serve multiple concurrent IOPs we must
use a pool of io_units. Currently we protect them with
spinlock, need to think if this is the best way to go.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Remove mutex around sysfs file creation

The mutex protects the list so take it only around
list_add.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: create xnbd_device sysfs

Add device state (read only) & delete (write only) attributes.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: change log level of incoming message

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
xnbd: Move xio_release_response to on_submit_answer

It is part of submit answer processing.

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