bootstd: Correct virtio block-device handling

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: a60f7a3e ("bootstd: Add a virtio bootdev")
Reported-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>