blockSD: Refactor matadataValidity

The previous code was confusing, using positive conditions to set the
falsy result; this patch make it clear.

Change-Id: Ia84c5c988ca6187df2fa13130e03b8447de7e89d
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25208
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>