Config: Add support for restoring using a timing CPU

Config: Add support for restoring using a timing CPU
Currently there is an assumption that restoration from a checkpoint will
happen by first restoring to an atomic CPU and then switching to a timing
CPU. This patch adds support for directly restoring to a timing CPU. It
adds a new option '--restore-with-cpu' which is used to specify the type
of CPU to which the checkpoint should be restored to. It defaults to
'atomic' which was the case before.