aio: Restore aio ring content

1)Dump/restore mmaped aio ring like any other private vma entry,
  with the only exception we do not predump it.
2)Create io context, set head and tail using write to /dev/null.
3)Copy aio ring restored in (1) to created in (2).
4)Remap (2) to address of (1).

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>