libsnapshot: MapPartitionWithSnapshot cleanup itself if failed

When we run MapPartitionWithSnapshot, intermediate devices aren't
cleaned up if the call fails. Hence, record these intermediate devices
we have created along the way using the new AutoDevices class. Upon
failure, the AutoDevices object will be destroyed, and all the
intermediate devices will be deleted from device mapper or image
manager. Upon success, AutoDevices::Release() makes sure the
intermediate devices aren't deleted.

Test: libsnapshot_test

Change-Id: Iff4c1297528288a27765c0224b67254b68c89776