hw/nvme: force nvme-ns param 'shared' to false if no nvme-subsys node

hw/nvme: force nvme-ns param 'shared' to false if no nvme-subsys node

Since commit 916b0f0b5264 ("hw/nvme: change nvme-ns 'shared' default")
the default value of nvme-ns param 'shared' is set to true, regardless
if there is a nvme-subsys node or not.

On a system without a nvme-subsys node, a namespace will never be able
to be attached to more than one controller, so for this configuration,
it is counterintuitive for this parameter to be set by default.

Force the nvme-ns param 'shared' to false for configurations where
there is no nvme-subsys node, as the namespace will never be able to
attach to more than one controller anyway.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>