This code, living in device.c for maximum generality, will read the partition
table from any disk that calls it. Ideally, each new device would have its own
private data. But that would mean having to callback to the driver to set each
of the partitions up. Therefore, I found it easier to convention that all
partitions in the same drive have the same private data. This makes some sense
if we consider that the hypervisors are usually agnostic about partitions, and
all of the addressing and communications go through a single entry point, which
is the disk.