snapuserd: Add support for Xor ops in snapuserd

This adds the ability to read the blocks corresponding to xor operations
from snapuserd. Xor Operations should be treated the same as copy
operations during the merge, but before their data is written to disk,
it must be xor'ed against the operation's data. For the purposes of
readahead, this acts as a copy op. Post readahead, it acts more like a
replace op.

Change-Id: I7d74bbdd43bfc5277ef4c8aade57dd375be9180a
Bug: 177104308
Test: vts_libsnapshot_test and cow_snapuserd_test