hw/nvme: revert CMIC behavior

hw/nvme: revert CMIC behavior

Commit cd59f50ab017 ("hw/nvme: always initialize a subsystem") causes
the controller to always set the CMIC.MCTRS ("Multiple Controllers")
bit. While spec-compliant, this is a deviation from the previous
behavior where this was only set if an nvme-subsys device was explicitly
created (to configure a subsystem with multiple controllers/namespaces).

Revert the behavior to only set CMIC.MCTRS if an nvme-subsys device is
created explicitly.

Reported-by: Alan Adamson <alan.adamson@oracle.com>
Fixes: cd59f50ab017 ("hw/nvme: always initialize a subsystem")
Reviewed-by: Alan Adamson <alan.adamson@oracle.com>
Tested-by: Alan Adamson <alan.adamson@oracle.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>