vhost: fix guest notification setting

vhost: fix guest notification setting

If rte_vhost_enable_guest_notification is called before
the virtqueue is ready, the configuration is lost.

This patch fixes this by saving the guest notification
enablement value requested by the application, and apply
it before the virtqueue is made ready to the application.

Fixes: 604052ae5395 ("net/vhost: support queue update")
Reported-by: Yinan Wang <yinan.wang@intel.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Yinan Wang <yinan.wang@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>