ghost-files: Save device and inode in image

Because we need to lookup for ghost files from
inotify system where we only have device/inode
as a key, we save dev/ino in ghost image entry.

Note we use in-kernel format for device to be
consistent with inotify and mount related
code base.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>