cmd: host: fix seg fault at "host info"

With the patch below applied, host_block_dev structure was switched
to be placed in platdata rather than priv. The command "host info"
must be aligned with this change. Otherwise, we will see "Segmentation
Fault."

Fixes: 8f994c86 ("sandbox: blk: Switch to use platdata_auto_alloc_size for the driver data")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>