vz: set something in disk driver name

Absent driver name attribute is invalid xml. Which in turn makes
unusable 'virsh edit' for example. The value does not make
much sense and ignored on input so nobody will hurt.

src/vz/vz_sdk.c

@@ -647,6 +647,9 @@ prlsdkGetDiskInfo(vzDriverPtr driver,
             VIR_FREE(disk->serial);
     }
 
+    if (virDomainDiskSetDriver(disk, "vz") < 0)
+        goto cleanup;
+
     ret = 0;
 
  cleanup: