x86/mm: Fix liveness of pages in grant copy operations

x86/mm: Fix liveness of pages in grant copy operations

We were immediately putting the p2m entry translation for grant
copy operations. This allowed for an unnecessary race by which the
page could have been swapped out between the p2m lookup and the actual
use. Hold on to the p2m entries until the grant operation finishes.

Also fixes a small bug: for the source page of the copy, get_page
was assuming the page was owned by the source domain. It may be a
shared page, since we don't perform an unsharing p2m lookup.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>