fs_mgr: libdm: Add support to list existing device mapper devices

Test: dmctl create system; dmctl list devices; dmctl delete system;
      dmctl list devices
Bug: 110035986

Change-Id: I4ae5d40041458421068976fa2a99c662c542a9a1
Signed-off-by: Sandeep Patil <sspatil@google.com>