Integrate snapuserd with dm-user ABI changes.

Create loopback device to simulate system_a/product_a partitions
to test IO path.

Bug: 168259959
Test: cow_snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I9f2a311d3eccfa20c82d0ebdb3e028ea3323a48d