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>

arch/x86/kvm/emulate.c

@@ -1817,6 +1817,10 @@ static int em_push_sreg(struct x86_emulate_ctxt *ctxt)
 	int seg = ctxt->src2.val;
 
 	ctxt->src.val = get_segment_selector(ctxt, seg);
+	if (ctxt->op_bytes == 4) {
+		rsp_increment(ctxt, -2);
+		ctxt->op_bytes = 2;
+	}
 
 	return em_push(ctxt);
 }