CLOUDSTACK-2029 zone wide primary storage support for cloudstack over vmware deployments

DB changes to support hypervisor specific zone wide storage pool.
Added method findZoneWideStoragePoolsByHypervisor to PrimaryStorageDaoImpl to find suitable zone wide storage pool of specific hypervisor type.
Added column 'hypervisor' to table storage_pool. This column can be NULL. Used/populated only for zone wide primary storage pools.