qcow: add support for rebuilding refcounts
qcow: add support for rebuilding refcounts

This adds the ability to regenerate the reference counts by walking all
of the L1/L2 tables and headers to find all reachable clusters.  This is
necessary for the next patch, which will use the reference count tables
to find unused clusters to reuse.

BUG=chromium:899273
TEST=cargo test -p cqow

Change-Id: I93dd00d381d8d33010fddfc10aa18ca32586e1f4
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1327821
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm