xnbd: Rename xnbd_destroy_xnbd_conn -> xnbd_destroy_conn

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Introduce xnbd_create_conn

Move connection creation to a routine.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Fix error flows in xnbd_session_create

Cleanup correctly and escelate proper return codes.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Remove xnbd_indexes decrementation from xnbd_destroy_device

This is needed to prevent crash in sysfs when removing and adding
new xnbd device.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
xnbd: Add dev_name attribute to xnbd_file struct

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