xnbd: Move declarations to xnbd header file

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Move sysfs routines to xnbd_sysfs.c

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: WIP Move all blk-mq related to xnbd_mq.c

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