frontend: Fix validation of UnitVmModel

Individual entity models were validated but the validate() method always
returned true since the validity of individual tabs was not taken into
account. This resulted in submitting invalid edits and subsequent
backend validation errors.

Change-Id: Iabba7f93a0541292a1a4c82b675ab0faf141b9b2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1128444
Signed-off-by: Martin Betak <mbetak@redhat.com>