vhost: fix virtio freeze due to missed interrupt

vhost: fix virtio freeze due to missed interrupt

Update of used->idx and read of avail->flags could be reordered.
Memory fence should be used to ensure the order, otherwise guest could see
a stale used->idx value after it toggles the interrupt suppression flag.
After guest sets the interrupt suppression flag, it will check if there
is more buffer to process through used->idx. If it sees a stale value,
it will exit the processing while host won't send interrupt to guest.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Reviewed-by: Luke Gorrie <luke@snabb.co>