nvme/039: avoid module loads for various transport types

nvme/039: avoid module loads for various transport types

The test case nvme/039 does not depend on nvme transport type and does
not require modules for the transport types. The _nvme_requires call in
requires() loads the modules and those modules are left unloaded after
the test case run. This causes failures of following nvmeof-mp test runs
with message:

  modprobe: FATAL: Module nvmet is in use.

To avoid the unnecessary module loads, remove _nvme_requires call.
Instead, just check existence of nvme command.

Fixes: 9accb5f86670 ("tests/nvme: add tests for error logging")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Alan Adamson <alan.adamson@oracle.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>