Increment page reference count for every host/device

Increment page reference count for every host/device
mapping created via a grant reference, rather than one
increment for all uses of a single grant reference.
This avoids a nasty situation in put_page_from_l1e()
where we cannot reliably determine whether a mapping was
created via a grant reference and so we must always
decrement the mapped page's reference count.

Signed-off-by: Keir Fraser <keir@xensource.com>