x86/emul: Correct the decoding of SReg3 operands

x86/emul: Correct the decoding of SReg3 operands

REX.R is ignored when considering segment register operands, and needs masking
out first.

While fixing this, reorder the user segments in x86_segment to match SReg3
encoding.  This avoids needing a translation table between hardware ordering
and Xen's ordering.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>