vm: Update LUN size when starting a vm

In order to handle resize of DirectLUN disks
(i.e. when a LUN volume is extended by user),
relevant data should be retrieved when invoking getVmStats.

Hence, added the following information to disks stats for
DirectLUN disks: truesize, apparentsize and lunGUID
(the sizes values already exist but are zeroed).
Using these values, the engine could keep the DB updated.

The following changes are introduced:
* hsm:
  appropriateDevice method now fetches LUN's device size
  using 'multipath -> getDeviceSize'.
* clientIF:
  For LUN devices, updated drive's truesize/apparentsize
  using hsm's getVolumeSize method.
* vm:
  _getDiskStats -> in order to identify each LUN
  (e.g. on engine side), added 'lunGUID' to dStats
  when vmDrive.GUID exists (applicable for DirectLUN disks).

Change-Id: Ia4988212f7f96078e774d2a4e7b5cd1681383cb0
Bug-Url: https://bugzilla.redhat.com/1026868
Signed-off-by: Daniel Erez <derez@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24235
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>