KVM: x86: Emulate push sreg as done in Core

According to Intel SDM push of segment selectors is done in the following
manner: "if the operand size is 32-bits, either a zero-extended value is pushed
on the stack or the segment selector is written on the stack using a 16-bit
move. For the last case, all recent Core and Atom processors perform a 16-bit
move, leaving the upper portion of the stack location unmodified."

This patch modifies the behavior to match the core behavior.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>