Determine guest mode status for metrics collection

Modification to the function IsGuestMode. It returns true only if the guestfs
partition is mounted and the file /var/run/state/logged-in exits. Previously
it did not check for this file existence.

BUG=chromium-os:15763
TEST=Log in in to guest mode right after installing recovery image and check
the absence of /home/chronos/prev_stats. Log out and log in as a user and
confirm the existence of this directory.

Change-Id: I29c66642ba590e882e6ec7a02732e818c87ac2ad
Reviewed-on: http://gerrit.chromium.org/gerrit/1586


Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Arkaitz Ruiz Alvarez <arkaitzr@google.com>