KVM: x86 emulator: introduce NoWrite flag

Instead of disabling writeback via OP_NONE, just specify NoWrite.

Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>