Changes to fix bad DMA handling by disk when using scatter gather page

Changes to fix bad DMA handling by disk when using scatter gather page
mappings (writes still unverified, and read could probably be looked over
a bit more too)

dev/ide_disk.cc:
dev/ide_disk.hh:
    Major changes to fix bogus handling of dma transfers.  The read seems to
    work pretty well, but the write is unverfied until we can get a disk
    image that actually tries to write.
dev/tsunami_pchip.cc:
    Change mask to properly mask 13 bits not 12

--HG--
extra : convert_revision : 08fe9bc32970e449cd045de479553a96a4e389d6