qemu: Forbid rx/tx_queue_size change explicitly
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1484230
    
    When updating a virtio enabled vNIC and trying to change either
    of rx_queue_size or tx_queue_size success is reported although no
    operation is actually performed. Moreover, there's no way how to
    change these on the fly. This is due to way we check for changes:
    explicitly for each struct member. Therefore it's easy to miss
    one.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>