x86emul: don't assume a memory operand

x86emul: don't assume a memory operand

Especially for x86_insn_operand_ea() to return dependable segment
information even when the caller didn't consider applicability, we
shouldn't have ea.type start out as OP_MEM. Make it OP_NONE instead,
and set it to OP_MEM when we actually encounter memory like operands.

This requires to eliminate the XSA-123 fix, which has been no longer
necessary since the elimination of the union in commit dd766684e7. That
in turn allows restricting the scope of override_seg to x86_decode().
At this occasion also make it have a proper type, instead of plain int.

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