virtio-pci: correctly set host notifiers for modern bar

Currently, during host notifier set. We only add eventfd for legacy
bar, this is not correct since:

- Non-transitional device does not have legacy bar, so qemu will crash
  since proxy->bar was not initialized.
- Modern device uses modern bar and notify cap to notify the device,
  we should add eventfd for proxy->notify.

So this patch fixes the above two issues by adding eventfd based on
whether legacy or modern device were supported.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

