Start defining the xenstore protocol in the Blkproto module

We want to make it easy to create stand-alone disk server
applications.
Add Protocol.{t,of_string,to_string}
Common functions to parse and print the disk info (sectors, sector size etc)
Add RingInfo.t which describes the shared memory ring exported by the frontend
Add RingInfo.to_string
Blkback.init now takes a RingInfo.t
RingInfo.keys is the list of keys needed to create a RingInfo.t
Correct typo in comment (VDISK_READONLY -> VDISK_CDROM)
Add State.{to,from}_assoc_list
Export xenstore keys used by unix toolstacks
Merge pull request #2 from djs55/xenstore-protocol

Add more of the xenstore protocol to blkproto