Use subquery filters for hosts if appropriate

refs #2934

(EL): Alternative service group subquery filter:

```php
$this->joinVirtualTable('services');
$query->joinVirtualTable('members');
return ['sgm.service_object_id', 'so.object_id'];
```

But I think the committed solution is better.

Signed-off-by: Eric Lippmann <eric.lippmann@icinga.com>

modules/monitoring/library/Monitoring/Backend/Ido/Query/HoststatusQuery.php

@@ -20,6 +20,11 @@ class HoststatusQuery extends IdoQuery
      */
     protected $groupOrigin = array('hostgroups', 'servicegroups', 'services');
 
+    protected $subQueryTargets = array(
+        'hostgroups'    => 'hostgroup',
+        'servicegroups' => 'servicegroup'
+    );
+
     /**
      * {@inheritdoc}
      */
@@ -291,4 +296,19 @@ class HoststatusQuery extends IdoQuery
                 'unhandled_service_count'
             ));
     }
+
+    protected function joinSubQuery(IdoQuery $query, $name)
+    {
+        if ($name === 'hostgroup') {
+            $query->joinVirtualTable('members');
+
+            return ['hgm.host_object_id', 'ho.object_id'];
+        } elseif ($name === 'servicegroup') {
+            $query->joinVirtualTable('services');
+
+            return ['s.host_object_id', 'ho.object_id'];
+        }
+
+        return parent::joinSubQuery($query, $name);
+    }
 }