Use the correct text VKey for HostResource's superordinateDomain (#608)

* Store the superordinateDomain reference as a VKey rather than Key

This is a reference to a Domain object, so we should store it as a VKey
in reference to the Domain table. This should not affect any business
logic, but rather will allow us to set up the SQL tables for
HostResource et al. properly.

core/src/main/resources/META-INF/persistence.xml

@@ -61,6 +61,7 @@
 
     <!-- Generated converters for VKey -->
     <class>google.registry.model.billing.VKeyConverter_BillingEvent</class>
+    <class>google.registry.model.domain.VKeyConverter_DomainBase</class>
     <class>google.registry.model.domain.token.VKeyConverter_AllocationToken</class>
     <class>google.registry.model.host.VKeyConverter_HostResource</class>
     <class>google.registry.model.contact.VKeyConverter_ContactResource</class>