hw/block/nvme: introduce nvme-subsys device

To support multi-path in QEMU NVMe device model, We need to have NVMe
subsystem hierarchy to map controllers and namespaces to a NVMe
subsystem.

This patch introduced a simple nvme-subsys device model.  The subsystem
will be prepared with subsystem NQN with <subsys_id> provided in
nvme-subsys device:

  ex) -device nvme-subsys,id=subsys0: nqn.2019-08.org.qemu:subsys0

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Tested-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
[k.jensen: added 'nqn' device parameter per request]
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>