x86: fix SS selector in SYSRET

According to my reading of the Intel documentation, the SYSRET instruction
is supposed to force the RPL bits of the %ss register to 3 when returning
to user mode. The actual sequence is:

SS.Selector <-- (IA32_STAR[63:48]+8) OR 3; (* RPL forced to 3 *)

However, the code in helper_sysret() leaves them at 0 (in other words, the "OR
3" part of the above sequence is missing). It does set the privilege level
bits of %cs correctly though.

This has caused me trouble with some of my VxWorks development: code that runs
okay on real hardware will crash on QEMU, unless I apply the patch below.

Signed-off-by: Bill Paul <wpaul@windriver.com>
Message-Id: <201503091548.01462.wpaul@windriver.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
