xen-blk{front/back}: remove broken FreeBSD extensions

The FreeBSD extension adds a new request type, called blkif_segment_block
which has a size of 112bytes for both i386 and amd64. This is fine on
amd64, since requests have a size of 112B there also. But this is not true
for i386, where requests have a size of 108B. So on i386 we basically
overrun the ring slot when queuing a request of type blkif_segment_block_t,
which is very bad.

Remove this extension (including a cleanup of the public blkif.h header
file) from blkfront and blkback.

Sponsored by: Citrix Systems R&D
Tested-by: cperciva