[virtio] Support VIRTIO_NET_F_IOMMU_PLATFORM

Since we don't enable IOMMU at all, we can then simply enable the
IOMMU support by claiming the support of VIRITO_F_IOMMU_PLATFORM.
This fixes booting failure when iommu_platform is set from qemu cli.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/virtio-net.c

 		( 1ULL << VIRTIO_NET_F_MAC ) |
 		( 1ULL << VIRTIO_NET_F_MTU ) |
 		( 1ULL << VIRTIO_F_VERSION_1 ) |
-		( 1ULL << VIRTIO_F_ANY_LAYOUT ) ) );
+		( 1ULL << VIRTIO_F_ANY_LAYOUT ) |
+		( 1ULL << VIRTIO_F_IOMMU_PLATFORM ) ) );
 	vpm_add_status ( &virtnet->vdev, VIRTIO_CONFIG_S_FEATURES_OK );
 
 	status = vpm_get_status ( &virtnet->vdev );

src/include/ipxe/virtio-ring.h

 #define VIRTIO_F_ANY_LAYOUT             27
 /* v1.0 compliant. */
 #define VIRTIO_F_VERSION_1              32
+#define VIRTIO_F_IOMMU_PLATFORM         33
 
 #define MAX_QUEUE_NUM      (256)