vdpa: add new callback get_vq_num_min in vdpa_config_ops

This callback is optional. For vdpa devices that not support to change
virtqueue size, get_vq_num_min and get_vq_num_max will return the same
value, so that users can choose a correct value for that device.
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/f4af5b0abd660d9a29ab6b2f67bd6df10284a230.1635493219.git.wuzongyong@linux.alibaba.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/vdpa.h

@@ -171,6 +171,9 @@ struct vdpa_map_file {
  * @get_vq_num_max:		Get the max size of virtqueue
  *				@vdev: vdpa device
  *				Returns u16: max size of virtqueue
+ * @get_vq_num_min:		Get the min size of virtqueue (optional)
+ *				@vdev: vdpa device
+ *				Returns u16: min size of virtqueue
  * @get_device_id:		Get virtio device id
  *				@vdev: vdpa device
  *				Returns u32: virtio device id
@@ -266,6 +269,7 @@ struct vdpa_config_ops {
 	void (*set_config_cb)(struct vdpa_device *vdev,
 			      struct vdpa_callback *cb);
 	u16 (*get_vq_num_max)(struct vdpa_device *vdev);
+	u16 (*get_vq_num_min)(struct vdpa_device *vdev);
 	u32 (*get_device_id)(struct vdpa_device *vdev);
 	u32 (*get_vendor_id)(struct vdpa_device *vdev);
 	u8 (*get_status)(struct vdpa_device *vdev);