Ensure that NVRAM is persisted when exiting the debugger after an abort

The SIGABRT handler is not invoked by the abort() call in the loguru
fatal handler, but either way it's not necessary (it does its own abort).

Switch to explicitly cleaning up the machine object, which as a side
effect in the destructor chain will persist the NVRAM.

main.cpp

@@ -176,7 +176,8 @@ int main(int argc, char** argv) {
         power_off_reason = po_enter_debugger;
         enter_debugger();
 
-        abort();
+        // Ensure that NVRAM and other state is persisted before we terminate.
+        delete gMachineObj.release();
     });
 
     // redirect SIGINT to our own handler