libdm: Add helper classes for loop control.

This change adds a LoopControl class for interacting with the loop
control device. In addition it provides a LoopDevice class for creating
temporary loop devices. This is aimed at being a building block for libdm
tests, so the current functionality is limited to attaching and
detaching file descriptors and finding free loop devices.

Bug: 110035986
Test: libdm_test gtest
Change-Id: Ice2891e3a44a037aff7b81c63ac793815640d582