Switch back to replying to requests in order

If we parallelise and permute the replies, the linux blkfront
is confused. It's not clear whether the protocol permits reply
reordering; it could work because each slot has a request id
(what would that be for other than to match replies to requests
in different slots?)

The problem is easily exposed in the VM using a 'fio' config:

[global]
rw=read
bs=128k
ioengine=libaio
direct=1
iodepth=16

[xvde]
filename=/dev/xvde

Signed-off-by: David Scott <dave.scott@eu.citrix.com>