backend: Fixing log print when vdsm version is not supported in cluster

When adding new host we verify its vdsm version and cluster
compatibility as part of HandleVdsVersionCommand. During that we
printed the same message for both compatibilities issue (vdsm version
and cluster level support). This patch separates the validation part with
two different prints.

Change-Id: Iacfb3875a0617f978b4453769a0b1c5704282fa5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=974101
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>