frontend: Fix console issues

This patch fixes
 - an NPEx in the console cache refreshing code,
 - loading consoles state from local storage on VmConsoles creation to
   ensure the initial state corresponds to local storage,
 - fixes regression from 04d7c8c37d7ce18f014e505cd5c46e25610d7 that
   causes problems with loading UserPortalListModel and
   UserPortalBasicListModel items when no pools were present.

Change-Id: I5219b77cd013744bec8a1336bfc578fca3ec4b2b
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>