x86: Fix GRANT_PTE_FLAGS.

x86: Fix GRANT_PTE_FLAGS.

Since page table entries created through e.g. GNTTABOP_map_grant_ref
are being passed through adjust_guest_l1e(), they must not generally
get _PAGE_USER set - this will be taken care of by adjust_guest_l1e(),
and it will ensure that these don't get _PAGE_GLOBAL set
inadvertently.

Due to the implied security aspect here (_PAGE_GLOBAL getting set on
kernel pages for x86-64), I'd like to ask that this also be applied to
older maintained branches.

At the same time, set _PAGE_NX for pte-s created for grants (as long
as hardware supports it), since it should be only data pages that remote
domains are being given access to.

Signed-off-by: Jan Beulich <jbeulich@novell.com>