fix #1317: check wizard form also on validitychange

fix #1317: check wizard form also on validitychange

with the commit
    40342aa6c4278497b9ec8c9cce8739f9b29c2e8f
we introduced a validator on the guestidselector, but did not
notice that the wizard checks the validity on the field change event,
but the selector gets valid/invalid in an api callback(so a little later)

with this patch, we now validate the field correctly with validate()
and also listen on the validitychange event to catch it

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>