qa/nvme_loop: fix an issue on ubuntu 18.04

qa/nvme_loop: fix an issue on ubuntu 18.04

The following command:

```
echo /dev/sda | tee /sys/kernel/config/nvmet/subsystems/sda/namespaces/1/device_path
```

makes nvme_loop fail because fascinatingly, it adds an unexpected newline.

See:
```
/dev/sda
/dev/sda

1
tee: /sys/kernel/config/nvmet/subsystems/sda/namespaces/1/enable: No such file or directory
/dev/sda
1
```

Other distros don't have the same behavior:

```
CentOS 8
/dev/sda
/dev/sda
1

Ubuntu 20.04
/dev/sda
/dev/sda
1
```

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit f8e22fb3da9bfbdc75d88beb66543716afb19511)