vz: support filesystem type volume

vz: support filesystem type volume

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: Olga Krishtal <okrishtal@virtuozzo.com>