wallet: Be able to unlock the wallet for migration

Since migration reloads the wallet, the wallet will always be locked
unless the passphrase is given. migratewallet can now take the
passphrase in order to unlock the wallet for migration.

test/functional/wallet_migration.py

@@ -403,7 +403,6 @@ class WalletMigrationTest(BitcoinTestFramework):
 
         wallet.encryptwallet("pass")
 
-        assert_raises_rpc_error(-15, "Error: migratewallet on encrypted wallets is currently unsupported.", wallet.migratewallet)
         # TODO: Fix migratewallet so that we can actually migrate encrypted wallets
 
     def run_test(self):