virtio-blk: Check return value of blk_aio_ioctl

Since commit 1dc936aa (virtio-blk: Use blk_aio_ioctl) we silently lose
the request if blk_aio_ioctl returns NULL (not implemented).

Fix it by directly returning VIRTIO_BLK_S_UNSUPP as we used to do.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>

[ kwolf: Fixed build error on win32 ]
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/block/virtio-blk.c

@@ -201,6 +201,7 @@ static int virtio_blk_handle_scsi_req(VirtIOBlockReq *req)
 #ifdef __linux__
     int i;
     VirtIOBlockIoctlReq *ioctl_req;
+    BlockAIOCB *acb;
 #endif
 
     /*
@@ -278,8 +279,13 @@ static int virtio_blk_handle_scsi_req(VirtIOBlockReq *req)
     ioctl_req->hdr.sbp = elem->in_sg[elem->in_num - 3].iov_base;
     ioctl_req->hdr.mx_sb_len = elem->in_sg[elem->in_num - 3].iov_len;
 
-    blk_aio_ioctl(blk->blk, SG_IO, &ioctl_req->hdr,
-                  virtio_blk_ioctl_complete, ioctl_req);
+    acb = blk_aio_ioctl(blk->blk, SG_IO, &ioctl_req->hdr,
+                        virtio_blk_ioctl_complete, ioctl_req);
+    if (!acb) {
+        g_free(ioctl_req);
+        status = VIRTIO_BLK_S_UNSUPP;
+        goto fail;
+    }
     return -EINPROGRESS;
 #else
     abort();