Fix runtime error when there are no NVM devices.
Fix runtime error when there are no NVM devices.

Signed-off-by: Paweł Szulik <5b8afb0391fe7d791c73575389ea38fc6a1723c3@intel.com>

linux-on-ibm-z/cadvisor,linux-on-ibm-z/cadvisor,linux-on-ibm-z/cadvisor