Add multi-page shared ring support to xen-blkfront/back

[Paul Durrant <paul.durrant@citrix.com>]

These patches add support for multi-page shared rings to xen-blkfront/back.
Shared rings must always be a power of 2 in size and so the size is controlled
by the new 'max_ring_page_order' module parameter of xen-blkback. This defaults
to 0 so operationally there should be no difference with these patches in place.
The maximum value of 'max_ring_page_order' is currently limited to 2
(i.e. 4 pages).
xen-blkfront will always use the largest shared ring size supported by the back
end. To override this and continue to use a single page ring, as before, set the
'allow_multi_page_ring' module parameter to 0.