tests: make api check support falsey values

the current vmApiTests assume truthy values in the replies.
The schema actually doesn't requires that, and falsey values
(e.g. empty lists) are actually valid.

This patch fixes the tests to support them.

Change-Id: I4f974b08589a077dedba3006b29f6bb29aee05ba
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29438
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Tested-by: Martin Sivák <msivak@redhat.com>