frontend: Fix Vm Dialog name validation

Add missing call to setValidTab upon validation of name, description and
comment properties.

Change-Id: If15c482953c48107d155d1c5e8206c675ff9fb23
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1124363
Signed-off-by: Martin Betak <mbetak@redhat.com>