KVM: x86 emulator: Any legacy prefix after a REX prefix nullifies its effect

This patch modifies the management of REX prefix according behavior
I saw in Xen 3.1.  In Xen, this modification has been introduced by
Jan Beulich.

http://lists.xensource.com/archives/html/xen-changelog/2007-01/msg00081.html



Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>