[libvirt] [PATCH] Fix segfault if virtual network has no bridge listed

The attached patch fixes a libvirtd segfault if we try to call virNetworkGetBridgeName on a virtual network with no bridge in its xml. This occurs if the network hasn't been started as long as the daemon has been running. This patch also cleans up an error path in qemu_conf which was overwriting the error returned by virNetworkGetBridgeName. Thanks, Cole