x86/emul: Fix backport of "x86/emul: Fix the decoding of segment overrides in 64bit...

x86/emul: Fix backport of "x86/emul: Fix the decoding of segment overrides in 64bit mode"

The logic in c/s b7dce29d is correct for master, but depends on c/s 216c29765
"x86emul: don't assume a memory operand", and in particular changed the type
of override_seg and the constant used to signify "no override".

When backported, the "no override in place" case causes x86_seg_none to be
used in place of x86_seg_ds, which has implications for the correctness of
emulated memory reads and writes.

Problems from this manifest as a reliable failure to boot shadow guests in the
following manner:

  (d3) [15145.989017] Press F12 for boot menu.
  (d3) [15145.989730]
  (d3) [15146.000273] Boot device: CD-Rom5814MB medium detected
  (d3) [15146.013234]  - success.
  (XEN) [15146.324244] paging.c:694:d0v3 Tried to do a paging op on itself.
  (XEN) [15204.719457] sh error: sh_remove_shadows(): can't find all shadows of mfn 112cd54 (shadow_flags=00002000)
  (XEN) [15204.719462] domain_crash called from common.c:2884
  (XEN) [15204.719466] Domain 3 (vcpu#0) crashed on cpu#31:

Other problems with regular MMIO emulation haven't been observed, but
shouldn't be ruled out.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>