.. instead of String. This imposes a type-level restriction that the
keys of the map are just hypervisor names.

Note about 'Arbitrary GroupDiskParams': Since GroupDiskParams and
ClusterHvParams were both synonyms for 'Container (Container JSValue)',
the Arbitrary instance worked for both. After fixing the type of
ClousterHvParams, its instance become different from GroupDiskParams,
hence the latter needs the addition.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>