cleanup also when sys loop device used

cleanup also when sys loop device used

some interfaces are added to make LoopDevice as
loop device manager:
register: if loop device existed, register it in
          LoopDevice, and benefit the cleanup
reg_atexit: not all loop device need this atexit,
            make it optional
now, such as 'loop0' will benefit the cleanup of
LoopDevice 'cause above interface

Change-Id: I110a1aaf0179d04a31f31f7ed54b2b75cdb6bc01
Signed-off-by: Gui Chen <gui.chen@intel.com>