[Design] nested partitions: Unify grub_partition and grub_disk

[phcoder]

Hello. I was looking into nested partitions issue and propose to unify 
grub_partition and grub_disk. In this case when a code calls 
grub_disk_read with a grub_disk_t which represents a partition it will 
correct the offset and call grub_disk_read with underlying partition or 
disk. In this way not only nested partitions become natural but also 
conceptually similar fields of total_sectors and len become unified. As 
it's an important design change I send this e-mail before coding it
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Regards
Vladimir 'phcoder' Serbinenko