qemuDomainAttachRNGDevice: do not access source.file randomly

We pass the source.file to qemuCheckCCWS390AddressSupport for
the purpose of reporting an error message without actually checking
that the rng device is of type VIR_DOMAIN_RNG_BACKEND_RANDOM.

Change it to a hardcoded "rng" string, which also avoids
referring to the device by a host-side attribute.

src/qemu/qemu_hotplug.c

@@ -2159,7 +2159,7 @@ qemuDomainAttachRNGDevice(virConnectPtr conn,
         }
     } else {
         if (!qemuCheckCCWS390AddressSupport(vm->def, rng->info, priv->qemuCaps,
-                                            rng->source.file))
+                                            "rng"))
             goto cleanup;
     }