libct/devices: change devices.Type to be a string

Possibly there was a specific reason to use a rune for this, but I noticed
that there's various parts in the code that has to convert values from a
string to this type. Using a string as type for this can simplify some of
that code.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

libcontainer/rootfs_linux.go

@@ -710,7 +710,7 @@ func mknodDevice(dest string, node *devices.Device) error {
 	case devices.FifoDevice:
 		fileMode |= unix.S_IFIFO
 	default:
-		return fmt.Errorf("%c is not a valid device type for device %s", node.Type, node.Path)
+		return fmt.Errorf("%s is not a valid device type for device %s", node.Type, node.Path)
 	}
 	dev, err := node.Mkdev()
 	if err != nil {