https://bugzilla.redhat.com/show_bug.cgi?id=1366989

QEMU added another virtio-net tunable [1]. It basically allows
users to set the size of RX virtio ring. But because virtio-net
uses two separate ring buffers to pass data from/to guest they
named it explicitly rx_queue_size. We should expose it in our XML
too.

1: http://lists.nongnu.org/archive/html/qemu-devel/2016-08/msg02029.htmlSigned-off-by: NMichal Privoznik <mprivozn@redhat.com>