KVM: x86 emulator: fix unlocked CMPXCHG8B emulation

When CMPXCHG8B is executed without LOCK prefix it is racy. Preserve this
behaviour in emulator too.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -1724,7 +1724,6 @@ static inline int emulate_grp9(struct x86_emulate_ctxt *ctxt,
 		       (u32) c->regs[VCPU_REGS_RBX];
 
 		ctxt->eflags |= EFLG_ZF;
-		c->lock_prefix = 1;
 	}
 	return X86EMUL_CONTINUE;
 }