COW partition creator uses DmSnapshotCowSizeCalculator

Instead of using heuristics on the calculation of the COW size, use the
class representing the internal structure of the COW device as generated
by the Linux kernel.
This simplifies the forecast of the required COW device size depending
on the write operations performed on the associated snapshot device.

Created also an additional test case for the COW size calculation.

Change-Id: If147001544988716430b36a4c75dfa7f34b4c8d9
Bug: 140835698
Test: libsnapshot_test
Signed-off-by: Alessio Balsini <balsini@google.com>