add path to nvme list

add path to nvme list

In SPDK project (spdk.io) it is possible to represent
devices using CUSE as character devices in "/dev/spdk/" directory.
No sysfs entries are generated for those devices.

nvme-cli default behavior is to scan the sysfs for entries on
newer kernels. Previously legacy_list() was used to determine
the topology, but that worked only on "/dev/" path.

To enable listing SPDK devices following changes were made:
- pass path parameter to list command
- before checking sysfs, check the path using legacy_list()
- expand arguments of scan_subsystems and related functions to accept the path

Example:
nvme list -d /dev/spdk/

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>