blkback: functorise over Activations, Xenstore and a backing store

This allows us to add more of the setup logic to the library. We
now have

make: to create the frontend/backend directories
run: to serve the requests for a particular device
request_close: to request a hot-unplug
destroy: to destroy the frontend/backend directories

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

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
release 1.1.0, update CHANGES

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #25 from djs55/megafunctor

Functorise over Activations, Xenstore and backing store