kv: rework example to expose pure KV

[Firobe]

Hi!

This PR does two things:

• pin notafs directly from the unikernel's config.ml so it doesn't need to be pinned manually (so that step is removed from the README)
• introduce a notafs_kv_rw function in config.ml that takes a block implementation (and a few other parameters, see the doc) and returns a kv_rw implementation, so that notafs-kv can be a real drop-in replacement for unikernels that already expect a KV (say, from a tar file)

A few thoughts:

• here, all that code is introduced in the example config.ml, but of course the goal would be to merge this code upstream in mirage. This can be a first draft towards that goal
• I change the example, but we could very well duplicate it (since this doesn't prevent people to directly use Block and the library, as done initialy) if you prefer
• we could have a notafs_kv_ro as well if needed
• currently notafs_kv_rw takes a format parameter to determine if the block should be formatted always, never or the first time. This could very well be a proper configure-time or run-time parameter as well
• the logic for connection could be slightly easier if Notafs.KV took the checksum implementation as its first argument instead of its second (we wouldn't need to define a type for checksums)