Prior to this patch, if a private nvme-ns device (that is, a namespace
that is not linked to a subsystem) is wired up to an nvme-subsys linked
nvme controller device, the device fails to verify that the namespace id
is unique within the subsystem. NVM Express v1.4b, Section 6.1.6 ("NSID
and Namespace Usage") states that because the device supports Namespace
Management, "NSIDs *shall* be unique within the NVM subsystem".

Additionally, prior to this patch, private namespaces are not known to
the subsystem and the namespace is considered exclusive to the
controller with which it is initially wired up to. However, this is not
the definition of a private namespace; per Section 1.6.33 ("private
namespace"), a private namespace is just a namespace that does not
support multipath I/O or namespace sharing, which means "that it is only
able to be attached to one controller at a time".

Fix this by always allocating namespaces in the subsystem (if one is
linked to the controller), regardless of the shared/private status of
the namespace. Whether or not the namespace is shareable is controlled
by a new `shared` nvme-ns parameter.

Finally, this fix allows the nvme-ns `subsys` parameter to be removed,
since the `shared` parameter now serves the purpose of attaching the
namespace to all controllers in the subsystem upon device realization.
It is invalid to have an nvme-ns namespace device with a linked
subsystem without the parent nvme controller device also being linked to
one and since the nvme-ns devices will unconditionally be "attached" (in
QEMU terms that is) to an nvme controller device through an NvmeBus, the
nvme-ns namespace device can always get a reference to the subsystem of
the controller it is explicitly (using 'bus=' parameter) or implicitly
attaching to.

Fixes: e5707685 ("hw/block/nvme: support for shared namespace in subsystem")
Cc: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: NKlaus Jensen <k.jensen@samsung.com>
Reviewed-by: NGollu Appalanaidu <anaidu.gollu@samsung.com>
Reviewed-by: NKeith Busch <kbusch@kernel.org>
Reviewed-by: NMinwoo Im <minwoo.im.dev@gmail.com>