fix Exception message error in apiTests module

Change-Id: Ic51b141047dc0490e50a050fd667f63d30f121f2
Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/11289
Reviewed-by: Dan Kenigsberg <danken@redhat.com>