Bump the size of the blockif scatter-gather list to 67.

The Windows virtio driver ignores the advertized seg_max
field and assumes the host can accept up to 67 segments
in indirect descriptors, triggering an assert in the bhyve
process.

No objection from: mav
Reviewed by: neel
Reported and tested by: Leon Dang (ldang@nahannisys.com)
MFC after: 2 weeks