cg: Use relative paths in cgroup dirs image

Before the patch cg tree section from cgroup00 test looked like this

{
	cnames: "name=zdtmtst"
	dirs: 	{
		path: "/subcg"
		children: 		{
			path: "/subcg/subsubcg"
			children: <empty>
			properties: <empty>
		}

		properties: <empty>
	}

}

this /subsg in the children is excessive. Turn this into directory names.
Now the section looks like

{
	cnames: "name=zdtmtst"
	dirs: 	{
		dir_name: "subcg"
		children: 		{
			dir_name: "subsubcg"
			children: <empty>
			properties: <empty>
		}

		properties: <empty>
	}

}

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>