Commit baade4d fixed a memory leak on failure, but in the process,
introduced a use-after-free on success, which can be triggered with:

1. set bandwidth with --live
2. query bandwidth
3. set bandwidth with --live

* src/qemu/qemu_driver.c (qemuDomainSetInterfaceParameters): Don't
free newBandwidth on success.
Reported by Hu Tao.