engine: Replacing search for vms by query

The following patch will replace the search for VMs which belong to some cluster, by query from DB.
The query is much more efficient than search , and easy to understand.
The patch contains:
1. Introducing new query
2. Replace all searches by new query
3. Removed class HighAvailableVmsDirector (The following class also uses a search query, but
   the class is not used anywhere, that's why I remove it instead of replacing search query)

Change-Id: I12cee2c66aed35ed357f2ee739232d95fe9fa0c1
Signed-off-by: Michael Kublin <mkublin@redhat.com>