remove checkpoint / restart code

remove checkpoint / restart code

The checkpoint / restart code is too experimental regarding
the current implementation in the kernel.
As there are several implementation and we don't know which
one will be merged upstream, I remove the code and add a few
parameters to checkpoint.

The checkpoint/restart function will be plugins in order to choose
the CR solutions. This approach will allow to switch from one
implementation to another without breaking anything.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>