Add support for blkio.weight_device

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

blkiodev/blkiodev.go

@@ -0,0 +1,15 @@
+package blkiodev
+
+import (
+	"fmt"
+)
+
+// WeightDevice is a structure that hold device:weight pair
+type WeightDevice struct {
+	Path   string
+	Weight uint16
+}
+
+func (w *WeightDevice) String() string {
+	return fmt.Sprintf("%s:%d", w.Path, w.Weight)
+}