libxl: fix net device detach

Chunyan sent a nice cleanup patch for libxlDomainDetachNetDevice

https://www.redhat.com/archives/libvir-list/2016-March/msg00926.html

which I incorrectly modified before pushing as commit b5534e53. My
modification caused network devices of type hostdev to no longer
be removed. This patch changes b5534e53 to resemble Chunyan's
original, correct patch.

src/libxl/libxl_driver.c

@@ -3449,12 +3449,14 @@ libxlDomainDetachNetDevice(libxlDriverPrivatePtr driver,
         goto cleanup;
     }
 
-    networkReleaseActualDevice(vm->def, detach);
-    virDomainNetRemove(vm->def, detachidx);
     ret = 0;
 
  cleanup:
     libxl_device_nic_dispose(&nic);
+    if (!ret) {
+        networkReleaseActualDevice(vm->def, detach);
+        virDomainNetRemove(vm->def, detachidx);
+    }
     virObjectUnref(cfg);
     return ret;
 }