nvme-cli: allow discover to address discovery controller by persistent name

nvme-cli: allow discover to address discovery controller by persistent name

To support discovery (connect/connect-all) to operate against a
persistent discovery controller, let the discovery controller to
be specified by its device node name rather than new connection
attributes.

Example:
  nvme connect-all ... --device=nvme5

Also centralize extraction of controller instance from the controller
name to a common helper.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>