Commit b1f416aa breaks vhost_net
because it always registers the virtio_pci_host_notifier_read() handler
function on the ioeventfd, even when vhost_net.ko is using the ioeventfd.
The result is both QEMU and vhost_net.ko polling on the same eventfd
and the virtio_net.ko guest driver seeing inconsistent results:

  # ifconfig eth0 192.168.0.1 netmask 255.255.255.0
  virtio_net virtio0: output:id 0 is not a head!

To fix this, proceed the same as we do for irqfd: add a parameter to
virtio_queue_set_host_notifier_fd_handler and in that case only set
the notifier, not the handler.

Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Tested-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>