x86: tighten filter on ptwr_do_page_fault()

x86: tighten filter on ptwr_do_page_fault()

Even not-so-recent Linux may, due to post-2.6.18 changes to the
process creation code, cause quite a number (depending on environment
and argument size) of faulting accesses to user space originating from
kernel mode. Generally those happen for non-present pages and would
lead to a nested page fault from guest_get_eff_l1e(). They can be
avoided by checking for PFEC_page_present as long as the guest isn't
running on shadow page tables.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Keir Fraser <keir@xen.org>