nvme-print-json: include vs for identify namespace

nvme-print-json: include vs for identify namespace

While the spec may allow for arbitrary use of vendor-specific
data, some implementations are simply strings.  JSON will
encode the string, allowing for safe use of control characters.

The only requirement is that the there be a null-byte in the
vendor-specific field terminating the string.  Data beyond the
null-byte will be ignored.

Signed-off-by: Chris Patterson <cpatterson@microsoft.com>
[dwagner: use d_json instead obj_add_str]
Signed-off-by: Daniel Wagner <dwagner@suse.de>