snapuserd: Add an extractor tool.

This is similar to inspect_cow --extract-to, except it uses snapuserd.
It is a diagnostic host tool and uses the tooling added for host
testing.

Usage: snapuserd_extractor -cow COW_FILE -base BASE_FILE -out OUT_FILE
                           -num_sectors NUM_SECTORS

Unlike inspect_cow, this supports xor/copy operations.

The extractor code is separated into a utility file so we can use it for
additional tests later on.

Bug: N/A
Test: manual test
Change-Id: Ib7509508cba45e6c3a0db8c75454e33c2a503e03