nvme: Search for specific subsysnqn in _find_nvme_loop_dev

nvme: Search for specific subsysnqn in _find_nvme_loop_dev

This ensures we find the correct nvme loop device if others exist on a
given system (which is generally not expected on test systems).

Additionally, this will be required in the upcomming test nvme/037 which
will have controllers racing with ones being destroyed.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>