libdm, libsnapshot: Add a virtual interface for DeviceMapper.

It is difficult to test libsnapshot failures because there is no way to
inject failure scenarios into the kernel's device-mapper. Exposing a
virtual interface will allow this kind of testing via dependency
injection.

Bug: 199785546
Test: builds, tests pass
Change-Id: I5add0c7e24f7fa2853114eaeda7c479eaa4e0552