hw/block/nvme: support namespace detach

Given that now we have nvme-subsys device supported, we can manage
namespace allocated, but not attached: detached.  This patch introduced
a parameter for nvme-ns device named 'detached'.  This parameter
indicates whether the given namespace device is detached from
a entire NVMe subsystem('subsys' given case, shared namespace) or a
controller('bus' given case, private namespace).

- Allocated namespace

  1) Shared ns in the subsystem 'subsys0':

     -device nvme-ns,id=ns1,drive=blknvme0,nsid=1,subsys=subsys0,detached=true

  2) Private ns for the controller 'nvme0' of the subsystem 'subsys0':

     -device nvme-subsys,id=subsys0
     -device nvme,serial=foo,id=nvme0,subsys=subsys0
     -device nvme-ns,id=ns1,drive=blknvme0,nsid=1,bus=nvme0,detached=true

  3) (Invalid case) Controller 'nvme0' has no subsystem to manage ns:

     -device nvme,serial=foo,id=nvme0
     -device nvme-ns,id=ns1,drive=blknvme0,nsid=1,bus=nvme0,detached=true

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>