core+ui: Add host notifications

This patch adds 3 new events to the notification service
- storage domain became inactive
- host became non-operational
- Engine failed to elect an SPM for a DC

Change-Id: Ie11d3d4e2c5e8b9a8096d742bc35cab82ff27bd0
Signed-off-by: Ofri Masad <omasad@redhat.com>