conf: implement XML parsing/formating for <dataStore> element of a storage <source>

conf: implement XML parsing/formating for <dataStore> element of a storage <source>

Introduce parsing and formatting of <dataStore> element. The <dataStore
represents a different storage volume meant for storing the actual
blocks of guest-visible data. The original disk source is then just a
metadata storage for any advanced features.

This currently works only for 'qcow2' images.

Signed-off-by: Nikolai Barybin <nikolai.barybin@virtuozzo.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>