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This is the first in a series of changes that are designed to
introduce better support for dynamic block devices.

It starts by defining a new Volume object which represents a storage
endpoint that knows how to mount, unmount, and format itself.  This
could be a filesystem directly on a partition, or it could be an
emulated FUSE filesystem, an ASEC, or an OBB.

These new volumes can be "stacked" so that unmounting a volume will
also unmount any volumes stacked above it.  Volumes that provide
shared storage can also be asked to present themselves (through bind
mounts) into user-specific mount areas.

This change also adds a Disk class which is created based on block
kernel netlink events.  Instead of waiting for partition events from
the kernel, it uses gptfdisk to read partition details and creates
the relevant Volume objects.

Change-Id: I0e8bc1f8f9dcb24405f5e795c0658998e22ae2f7