Generalise RingInfo record to include multi-page rings.

Note that blkback is not advertising a max-ring-page-order yet.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Don't assume that Io_pages are pre-zeroed; explicitly zero the ring

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge branch 'multi-page-ring' of github.com:djs55/ocaml-xen-block-driver into multi-page-ring

Conflicts:
	lib/blkproto.ml