x86_emulate: fix flag setting for 8-bit signed multiplication

x86_emulate: fix flag setting for 8-bit signed multiplication

We really need to check for a signed overflow of 8 bits, while the
previous check compared the sign-extended 8-bit result with the
zero-extended 16-bit one (which was wrong for all negative results).

Once at it
- also adjust the 16-bit comparison for symmetry
- improve the 8-bit multiplication (no need to zero-extend to 32-bits
  the sign-extended to 16 bits original 8-bit value)
- fold both signed multiplication variants

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