Add root field in State

`root` is path to the container's rootfs which can be useful information
to hook, and the official sample runtime implementation--runc--has
already added this field as payload to hook, we should add this to the
spec.

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

specs-go/state.go

@@ -10,6 +10,8 @@ type State struct {
 	Status string `json:"status"`
 	// Pid is the process ID for the container process.
 	Pid int `json:"pid"`
+	// Root is the path to the container's rootfs directory
+	Root string `json:"root"`
 	// BundlePath is the path to the container's bundle directory.
 	BundlePath string `json:"bundlePath"`
 	// Annotations are the annotations associated with the container.