fs_mgr: libdm: add support to create and delete device mapper devices.

Test: dmctl create system; dmctl delete system
Test: verify that ueventd creates /dev/block/dm-X and verify the dm
device name from /sys/block/dm-X/dm/name
Bug: 110035986

Change-Id: I2a08e2ea7007c0c13fe64d444f0d6618784edae7
Signed-off-by: Sandeep Patil <sspatil@google.com>