more strict KPI for vm_pager_get_pages()

[Gleb Smirnoff]

  Hi!

  I'm splitting the patch into a serie. This is step 1:

Pagers are responsible to update the array of pages in
case if they replace pages in an object. All array entries
must be valid, if pager returns VM_PAGER_OK.

Note: the only pager that replaces pages is sg_pager, and
it does that correctly.

-- 
Totus tuus, Glebius.
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