lguest: fix descriptor corruption in example launcher

1d589bb16b825b3a7b4edd34d997f1f1f953033d "Add serial number support
for virtio_blk, V4a" extended 'struct virtio_blk_config' to 536 bytes.
Lguest and S/390 both use an 8 bit value for the feature length, and
this change broke them (if the code is naive).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: John Cooper <john.cooper@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>