qemu: report error on attempt to live change virtio-net queues

qemu: report error on attempt to live change virtio-net queues

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1029732

The BZ asked for the capability to change the number of queues used by
a virtio-net device while the device is in use. Because the number of
queues can only be set at the time the device is created, that isn't
possible. However, libvirt also shouldn't be silently reporting
success when someone tries to change the number of queues. So this
patch flags that as an error (just as attempts to change any of the
other virtio-specific parameters already do).