libsnapshot: Add support for Xor ops in Cow Format

This adds support for Xor Ops in the Cow Format. These represent store
possibly compressed data which must be xor'ed against the given section
of source data to be interpreted as a block in the new image. The cow
reader and writer do not have access to this data, so they assume the
provider and user of the data will handle the xor-ing.

Bug: 177104308
Test: cow_api_test (ReadWriteXor)
Change-Id: I7a313d2643773d6b81a878a3e5bc87e3bdfc387b