misc: fix minor bugs (#906)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change

Fix hang when a DLL using the SDK calls `log_fatal` (deadlock)

Fix spicecfg crashing if API server fails to bind (make websocket
teardown conditional)

## Testing

src/spice2x/sdk/sdk.cpp

@@ -225,6 +225,7 @@ sdk_log(
             log_warning(facility_str.c_str(), "{}", message);
             break;
         case SPICE_SDK_LOG_LEVEL_FATAL:
+            lock.unlock();
             log_fatal(facility_str.c_str(), "{}", message);
             break;
         default: