libopalioctl: getDiskSerialNo: support NVME devices

In addition to device nodes supporting HDIO_* calls, also support NVME devices using the nvme_admin_identify command.

This is implemented in the dumbest way possible, namely by short-circuiting through each available serial-number-retrieving method in turn. If there is another way to identify a device node's driver purely from a file descriptor, that would be preferable.

This has been tested on a real drive and seems to work nicely.