vhost: fix invalid downcast

vhost: fix invalid downcast

virtio_queue_get_desc_addr returns 64-bit hwaddr while int is usually 32-bit.
If returned hwaddr is not equal to 0 but least-significant 32 bits are
equal to 0 then this code will not actually stop running queue.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Acked-by: Jia He <hejianet@gmail.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>