libsnapshot:snapuserd: Fix cow_snapuserd_test.

This will test the IO operation
between dm-snapshot/dm-user/snapuserd daemon. This test was broken
after cow format and daemon related changes.

Test: cow_snapuserd_test

Bug: 168311203
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I3e5f435332957a2549fd029c84efe13efb98b909