x86/tdx: Make VirtIO use DMA API in TDX guest

VirtIO for KVM is a primary way to provide IO for TD guest. All memory
that used for communication with the host has to be marked as shared.

The easiest way to archive that is to use DMA API that already knows how
to deal with shared memory.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

drivers/virtio/virtio_ring.c

@@ -255,6 +255,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
 	if (xen_domain())
 		return true;
 
+	if (is_tdx_guest())
+		return true;
+
 	return false;
 }