gnttab: set page refcount for copy-on-grant-transfer

gnttab: set page refcount for copy-on-grant-transfer

Commit 5cc77f9098 ("32-on-64: Fix domain address-size clamping,
implement"), which introduced this functionality, took care of clearing
the old page's PGC_allocated, but failed to set the bit (and install the
associated reference) on the newly allocated one. Furthermore the "mfn"
local variable was never updated, and hence the wrong MFN was passed to
guest_physmap_add_page() (and back to the destination domain) in this
case, leading to an IOMMU mapping into an unowned page.

Ideally the code would use assign_pages(), but the call to
gnttab_prepare_for_transfer() sits in the middle of the actions
mirroring that function.

This is XSA-284.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>