This patch enables VirtualDataStoreZabbix to access all DataStore instances.
To do this, 'DataStoreZabbix m_dataStoreZabbix' was replaced into
'DataStoreManager that concers'.

And VirtualDataStoreZabbix now provides add() to add DataStore instance
in the DataStoreManager form other classes. Actually, this is just to
pass the argument to DataStoreManager::add(). so this may not be cool.

<< Tasks to do in the near future >>
(P) create DataStoreZabbix instance in mainRoutine().
(Q) remove code to craete ArmZabbixAPI instance in ArmController.
    The ArmZabbixAPI instance will be created in DataStoreZabbix,
    When the above (P) is implemented.
(R) pass command line argument for ArmZabbiXAPI such as IP address etc.
    to DataSotreZabbix.

<< Next milestone >>
showing a 'Overview' of ZABBIX PHP UI. The assumed design of
the data stream is the following.

[Zabbix Server] =>
  (A) ArmZabbAPI -> (B) DataStoreZabbix -> (C) VirtualDataStoreZabbix
    -> (D) SQLProcessorZabbix -> (D) FaceMySQLWorker  => ZABBIX PHP client

<< Next next milestone >>
Merge data from two zabbix servers at (C) VirtualDataStoreZabbix