include: drivers: virtio: fix doxygen groups

Virtio Doxygen documentation is now folded under Device Driver API group
like it should be.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

include/zephyr/drivers/virtio.h

@@ -16,10 +16,10 @@ extern "C" {
 /**
  * @brief Virtio Interface
  * @defgroup virtio_interface Virtio Interface
+ * @ingroup io_interfaces
  * @{
  */
 
-
 /**
  * Callback used during virtqueue enumeration
  *

include/zephyr/drivers/virtio/virtqueue.h

@@ -22,6 +22,7 @@ extern "C" {
 /**
  * @brief Virtqueue Interface
  * @defgroup virtqueue_interface Virtqueue Interface
+ * @ingroup virtio_interface
  * @{
  */