libsnapshot: chunk iov writes

Currently if our iov that we are trying to write is greater than 1024
our write will fail with error "INVALID ARGUMENT". This is because
pwritev() system call takes a max input size of IOV_MAX (which is device
dependant).

With our increased cache size of 1mb or maybe even more (or if user
configures batch size to be large), our write size could be greater than
IOV_MAX, and will fail with an unhelpful error. We should chunk these
writes to ensure they succeed.

Bug: 322279333
Test: cow_api_test + manual testing with large iov write sizes
Change-Id: Ia1fb53cbfc743cfcdfc7256ff9df075ad0c2dd38