KVM: x86 emulator: fix cli/sti instruction emulation

If IOPL check fail, the cli/sti emulate GP and then we should
skip writeback since the default write OP is OP_REG.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>