Add Devices abstraction to CUDA image

This change adds a Devices abstraction to the CUDA image utilities. This
allows for checking whether a devices is selected, for example.

Signed-off-by: Evan Lezar <elezar@nvidia.com>

internal/modifier/mofed.go

@@ -43,7 +43,7 @@ func NewMOFEDModifier(logger *logrus.Logger, cfg *config.Config, ociSpec oci.Spe
 		return nil, err
 	}
 
-	if devices := image.DevicesFromEnvvars(visibleDevicesEnvvar); len(devices) == 0 {
+	if devices := image.DevicesFromEnvvars(visibleDevicesEnvvar); len(devices.List()) == 0 {
 		logger.Infof("No modification required; no devices requested")
 		return nil, nil
 	}