Use NATIVE MAX ADDRESS EXT to retrieve last block for LBA48. Also check not only
for enabled LBA48 support but for the 'host protected area' bit before using the
LBA48 version. This is because the high order byte (HOB) data retrieval is
broken in Qemu.
Fixes#761.
The block count in DMA requests is limited to 8 bit. Therefore,
if a client requests more than 255 blocks in a single packet request,
split the request in a loop.
