persist.go: saveInterface: loadInterface: change to use inf.spec.Id as id of PersistInterface
persist.go: saveInterface: loadInterface: change to use inf.spec.Id as id of PersistInterface

This change will fix the issue of restore.
And change var name to iId to make the code clear in LoadXPod and savePod.

Signed-off-by: Hui Zhu <3dff0dee05fafb3e3012a2d92c6bc151ff61663b@hyper.sh>

hyperhq/hyperd,laijs/hyper,laijs/hyper,hyperhq/hyper,laijs/hyper,laijs/hyperd,hyperhq/hyper,hyperhq/hyper,hyperhq/hyper,laijs/hyper,laijs/hyperd,hyperhq/hyperd,hyperhq/hyperd,laijs/hyperd