The __schema_name__ is used for validate method.
Then we need to use it in virt-xml-validate command.
And the param in virt-xml-validate for this type is named "storagevol"

Below is the all types supported by virt-xml-validate:
```
       Valid schema names currently include

       "domain"
           The schema for the XML format used by guest domains configuration

       "network"
           The schema for the XML format used by virtual network configuration

       "storagepool"
           The schema for the XML format used by storage pool configuration

       "storagevol"
           The schema for the XML format used by storage volume descriptions

       "nodedev"
           The schema for the XML format used by node device descriptions

       "capability"
           The schema for the XML format used to declare driver capabilities
```