blkback: avoid splitting requests into individual pages

We want to pass big requests down the stack, to reduce the impact
of per-request overheads (context-switching, enqueueing, dequeueing)

The backend implementation signature is now more like V1_LWT.BLOCK:
read and write take an offset and a list of buffers. Every request
is transformed into one of these, one buffer per segment. Hence
(for now, without indirect descriptors) each request has maximum
size 44KiB.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>