img: Warn about absolute paths for parent images dir

When working with mntns, the absolute path in parent symlink will
not be open-able on restore. However, completely banning this case
is not good.

Affects #116
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/image.c

@@ -415,6 +415,10 @@ int open_image_dir(char *dir)
 			pr_perror("Can't link parent snapshot");
 			goto err;
 		}
+
+		if (opts.img_parent[0] == '/')
+			pr_warn("Absolute paths for parent links "
+					"may not work on restore!\n");
 	}
 
 	return 0;