Heinz points out that adding buffers to a broken virtqueue (which
should "never happen") still works. Failing allows drivers to detect
and complain about broken devices.
Now drivers are robust, we can add this extra check.
Reported-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>