libdm: Add helpers for getting the device number of a dm device.

device-mapper will accept a "major:minor" string in place of a path for
any target parameter that specifies another block devie. This is useful
to have a helper for, since it lets us avoid a call to WaitForFile().

Bug: 134536978
Test: manual test
Change-Id: I13fd6699dd5d405bfa676830825b006b7810aa0f