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: I7b0e7e8844332cca3fa67611b90fada3c5ba0eeb
Reviewed-on: http://gerrit.chromium.org/gerrit/1647


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