KVM: MMU: Mark sp mmio cached when creating mmio spte

This will be used not to zap unrelated mmu pages when creating/moving
a memory slot later.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>