virtio-net: enable ioeventfd even if vhost=off

virtio-net-pci does not enable ioeventfd for historical reasons (and
nobody ever checked whether it should be revisited).  Note that other
backends do enable ioeventfd for virtio-net.

However, it has a major effect on performance.  On Windows, throughput is
_multiplied_ by 2 or 3 on TCP_STREAM (on small packets it is "only" a 30%
improvement) and a little less so on TCP_MAERTS albeit still very much
statistically significant.  Latency also has a single digit improvement.

This is not visible when using vhost, which forces ioeventfd=on, but it
is substantial without vhost.  In addition, also on Windows and with the
RHEL 7.3 kernel, APICv seems to slow down virtio-net performance a bit,
but the penalty with this patch goes from -25% to -7%.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
