core: StorageDomain - containsUnregisteredEntities flag

* Added 'containsUnregisteredEntities' flag to StorageDomain
  business entity for indicating an existence of unregistered
  VMs/Templates in the domain.
* Added a relevant column in storage domains views.
* Added tests to StorageDomainDAOTest.
* Created an index for 'storage_domain_id' column in
  'unregistered_ovf_of_entities' table.

Change-Id: I436b81634786de1c56fb3a89358ee9837e82307b
Signed-off-by: Daniel Erez <derez@redhat.com>