ghost: Remove ghost files if restore fails

Issue #18. When restore fails ghost files remain there. And
to remove them we have to know their list, paths to original
files (to construct the ghost name) and the namespace ghost
lives in.

For the latter we keep the restore task namespace at hands
till the final stage and setns into it to kill ghosts.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>