vm: added missing GUID slot in Drive class

Following commit I6e8dadabdd02d3b44606f215c4bc7b7e306a591a,
added missing 'GUID' slot in VM -> Drive class
(needed for the functionality introduced in commit
Ia4988212f7f96078e774d2a4e7b5cd1681383cb0).

In addition, verified 'GUID' key existence on _getDiskStats.

Change-Id: I2c64375138061e4310188baddbe114251ac157cc
Related-To-Bug-Url: https://bugzilla.redhat.com/1026868
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24418
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>