qcow: Add refcounting helper
qcow: Add refcounting helper

The refcounting helper breaks out management of the refcounts and
caching the refcount blocks.

Change-Id: I6e75fbe0eb47277ccf7a93af026b5020089875db
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1207452
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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