Vz containers are able to use ploop volumes from storage pools
to work upon.

To use filesystem type volume, pool name and volume name should be
specifaed in <source> :
   <filesystem type='volume' accessmode='passthrough'>
      <driver type='ploop' format='ploop'/>
      <source pool='guest_images' volume='TEST_POOL_CT'/>
      <target dir='/'/>
   </filesystem>

The information about pool and volume is stored in ct dom configuration:
<StorageURL>libvirt://localhost/pool_name/vol_name</StorageURL>
and can be easily obtained via PrlVmDevHd_GetStorageURL sdk call.

The only shorcoming: if storage pool is moved somewhere the ct
should be redefined in order to refresh the information aboot path
to root.hdd
Signed-off-by: NOlga Krishtal <okrishtal@virtuozzo.com>