testNodeDeviceMockCreateVport: Don't call public APIs
    
    This function is calling public APIs (virNodeDeviceLookupByName
    etc.). That requires the driver lock to be unlocked and locked
    again. If we, however, replace the public APIs calls with the
    internal calls (that public APIs call anyway), we can drop the
    lock/unlock exercise.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>