psbt: add the full transaction for the utxo in a psbt

We need this for dual funding, since the interactive tx construction
protocol requires the full tx to send places. We add it to all PSBTs (if
we have it), here

wallet/reservation.c

@@ -234,6 +234,7 @@ static bool inputs_sufficient(struct amount_sat input,
 }
 
 static struct wally_psbt *psbt_using_utxos(const tal_t *ctx,
+					   struct wallet *wallet,
 					   struct utxo **utxos,
 					   const struct ext_key *bip32_base,
 					   u32 nlocktime,
@@ -247,6 +248,7 @@ static struct wally_psbt *psbt_using_utxos(const tal_t *ctx,
 
 	for (size_t i = 0; i < tal_count(utxos); i++) {
 		u32 this_nsequence;
+		struct bitcoin_tx *tx;
 
 		if (utxos[i]->is_p2sh) {
 			bip32_pubkey(bip32_base, &key, utxos[i]->keyindex);
@@ -292,6 +294,14 @@ static struct wally_psbt *psbt_using_utxos(const tal_t *ctx,
 						      psbt->num_inputs - 1,
 						      &asset);
 		}
+
+		/* If we have the transaction for this utxo,
+		 * add it to the PSBT as the non-witness-utxo field.
+		 * Dual-funded channels and some hardware wallets
+		 * require this */
+		tx = wallet_transaction_get(ctx, wallet, &utxos[i]->txid);
+		if (tx)
+			psbt_input_set_utxo(psbt, i, tx->wtx);
 	}
 
 	return psbt;
@@ -327,7 +337,8 @@ static struct command_result *finish_psbt(struct command *cmd,
 			*locktime -= pseudorand(100);
 	}
 
-	psbt = psbt_using_utxos(cmd, utxos, cmd->ld->wallet->bip32_base,
+	psbt = psbt_using_utxos(cmd, cmd->ld->wallet, utxos,
+				cmd->ld->wallet->bip32_base,
 				*locktime, BITCOIN_TX_RBF_SEQUENCE);
 
 	/* Add a fee output if this is elements */