[paver] Re-write paver allocation under low-space constraints

- Pre-calculate necessary space for FVM image, validating
the header separately from actually allocating.
- Issue a visible message to the user if the incoming image
is too large to fit on the device.
- Re-image the FVM (destroying prior data) if the incoming
image cannot fit alongside the old image.
- Update the paver logging prefix from "install-disk-image"
to "paver"; this fits better on smaller screens, and is
more colloquial.

Test: Paving manually on Acer, Astro. Also, re-paving with low space.

Change-Id: Id94e1b609386364891e48eb37b9a3746575770c7

[disk-pave] Rewrite paver

An additional abstraction layer of a "device partitioner" interface was
added to the paver to allow for non-GPT device implementation to be
added more easily.

Tested:
Manually tested paving on pixelbook and nuc.
Tested wipe command followed by paving.
Tested paving to same device twice in a row.

Change-Id: I6b0d79c4702e20342e60125d15832e7fbb0b39fe