Force Interpreter VM when VM tracing requested (moved from State to Executive).

Fixes https://github.com/ethereum/cpp-ethereum/issues/2585.

libethereum/Executive.cpp

@@ -325,7 +325,8 @@ bool Executive::go(OnOpFunc const& _onOp)
 #endif
 		try
 		{
-			auto vm = VMFactory::create();
+			// Create VM instance. Force Interpreter if tracing requested.
+			auto vm = _onOp ? VMFactory::create(VMKind::Interpreter) : VMFactory::create();
 			if (m_isCreation)
 			{
 				auto out = vm->exec(m_gas, *m_ext, _onOp);

libethereum/State.cpp

@@ -1167,11 +1167,7 @@ ExecutionResult State::execute(LastHashes const& _lh, Transaction const& _t, Per
 	ctrace << toHex(e.t().rlp());
 #endif
 	if (!e.execute())
-	{
-		if (onOp)
-			VMFactory::setKind(VMKind::Interpreter); // force interpreter if a trace is wanted
 		e.go(onOp);
-	}
 	e.finalize();
 
 #if ETH_PARANOIA