libdm: Implement GetDmDevicePathByName().

This change implements the ability to get the path of a block device
given a device-mapper device name. In addition, dmctl now has a
"getpath" command to perform this query, as a shortcut for searching
through /sys/block/*/dm/name.

Bug: 110035986
Test: N/A
Change-Id: I9ebd824fa800004f591fc02fc1b1950e0c7fba65