plugins/spdk: plugin to list SPDK controllers

plugins/spdk: plugin to list SPDK controllers

Since libnvme and nvme-cli(2.0) are continuing as separate projects,
and libnvme use sysfs exclusively to gather information about NVMe
devices, legacy paths that allow specifying a custom directory ('-d`)
is no longer available.

SPDK, as a userspace driver, does not create a sysfs entry but makes it
possible to expose such devices via NVMe-cuse and since it behaves as
character device, most commands should be intact, except
listing devices and subsystems.

This patch adds SPDK devices to the nvme_root topology tree, which
allows for keeping functionality where both SPDK and kernel devices
are printed.

    $ nvme spdk list

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