guestagent: Use new style class

Using old style class is a trap for innocent developers. They do not
support properties in a nasty way, and should be forbidden in vdsm code.

Change-Id: I138ded6cb33d7927b2701606698aaf95ebf39742
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34624
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>