hw/block/nvme: support Identify NS Attached Controller List

Support Identify command for Namespace attached controller list.  This
command handler will traverse the controller instances in the given
subsystem to figure out whether the specified nsid is attached to the
controllers or not.

The 4096bytes Identify data will return with the first entry (16bits)
indicating the number of the controller id entries.  So, the data can
hold up to 2047 entries for the controller ids.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Tested-by: Klaus Jensen <k.jensen@samsung.com>
[k.jensen: rebased for dma refactor]
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>