x86: Use the right error code when testing for spurious pagefaults

x86: Use the right error code when testing for spurious pagefaults

Shadowed PV domains may take pagefaults with PFEC_reserved_bit bit
set, which are then turned into page-not-present faults by the shadow
code. Since that changes the error code in the regs structure, we need
to remember the old error code when we later check for spurious page
faults or we'll get false positives.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>