virtio_net: disable cb aggressively

There are currently two cases where we poll TX vq not in response to a
callback: start xmit and rx napi.  We currently do this with callbacks
enabled which can cause extra interrupts from the card.  Used not to be
a big issue as we run with interrupts disabled but that is no longer the
case, and in some cases the rate of spurious interrupts is so high
linux detects this and actually kills the interrupt.

Fix up by disabling the callbacks before polling the tx vq.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>