libdm: compute percentage of snapshot-merge

Computes the merge completion percentage from the sector information.

Provided test for the function.

Change-Id: I64d83baa0478f9e6969636ee067174910d9b8e03
Bug: N/A
Test: dm_test
Signed-off-by: Alessio Balsini <balsini@google.com>
3 files changed