vdpa: support exposing the count of vqs to userspace

- GET_VQS_COUNT: the count of virtqueues that exposed

Signed-off-by: Longpeng <longpeng2@huawei.com>
Link: https://lore.kernel.org/r/20220315032553.455-4-longpeng2@huawei.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Longpeng &amp;lt;&lt;a href="mailto:longpeng2@huawei.com" <target="_blank"&gt;longpeng2@huawei.com&lt;/a&gt;&amp;gt;&lt;br>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>