vdsm: check attribute not object type

Checking existance attribute is more flexible than checking object
type.


Change-Id: I19643f9b8a68a45d361492cdd5075b52616cd4a7
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/29874
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>