plugins/spdk: plugin to list SPDK subsystems

plugins/spdk: plugin to list SPDK subsystems

Since `nvme-cli` start using `sysfs` to gather information about
subsystems, SPDK devices that are represented using CUSE can
no longer be listed.

Support for the JSON format is disabled cause `nvme_ctrl_get_state` use
sysfs to get value, causing segmentation fault when it's missing.
It could be solved by providing an empty string instead of a NULL pointer.

This patch adds SPDK devices to the nvme_root topology tree,

     $ nvme spdk list-subsys

Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>