cloning: we won't unshare a gfn which is not yet accessed by VM

This improves performance a little
cloning: add unshare bitmap to record which gfn is unshared

Here unshared also means a gfn is already accessed, so its stage2
entry is valid.
cloning: add usage of unshare_bitmap

After stage2_set_pte, we set it. Before unsharing, we check it.