At present virtio tries to attach QEMU services to a bootdev device, which
cannot work. Add a check for this.
Also use bootdev_setup_sibling_blk() to create the bootdev device, since
it allows the correct name to be used and bootdev_get_sibling_blk() to
work as expected.
The bootdev is not created on sandbox since it does have a real virtio
device and it is not possible to read blocks.
Signed-off-by: Simon Glass <sjg@chromium.org> Fixes: a60f7a3e35b ("bootstd: Add a virtio bootdev") Reported-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>