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

@@ -19,11 +19,12 @@ extern "C" {
  * https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.pdf
  */
 
- /**
-  * @brief Virtqueue Interface
-  * @defgroup virtqueue_interface Virtqueue Interface
-  * @{
-  */
+/**
+ * @brief Virtqueue Interface
+ * @defgroup virtqueue_interface Virtqueue Interface
+ * @ingroup virtio_interface
+ * @{
+ */
 
 /**
  * used in virtq_desc::flags, enables chaining descriptor via virtq_desc::next