[XEN] Fix the x86 emulator to safely fail when it turns out the

[XEN] Fix the x86 emulator to safely fail when it turns out the
faulting memory access was to/from an implicit memory operand
(typically either an instruction fetch or stack access).
Rationalise use of macros for page fault error code flags.
This patch supercedes the fix in changeset 11242.
Signed-off-by: Keir Fraser <keir@xensource.com>