libxl: Remove unnecessary driver locking

Now that most fields of libxlDriverPrivate struct are immutable
or self-locking, there is no need to acquire the driver lock in
much of the libxl driver.

src/libxl/libxl_conf.c

@@ -1110,7 +1110,12 @@ error:
 libxlDriverConfigPtr
 libxlDriverConfigGet(libxlDriverPrivatePtr driver)
 {
-    return virObjectRef(driver->config);
+    libxlDriverConfigPtr cfg;
+
+    libxlDriverLock(driver);
+    cfg = virObjectRef(driver->config);
+    libxlDriverUnlock(driver);
+    return cfg;
 }
 
 virCapsPtr