c/r: initialize migrate_opts properly

c/r: initialize migrate_opts properly

The commit "c/r: add support for CRIU's --action-script" breaks
lxc-checkpoint on the command-line. It produces errors like:

 sh: $'\260\366\b\001': command not found

and then it fails. src/lxc/criu.c expects migrate_opts->action_script to
be either NULL, then it is ignored, or to actually contain the name of
an action scripts.

As the struct migrate_opts has not static storage is has to be explicitly
initialized or the value of the structure's members is indeterminate.

Signed-off-by: Adrian Reber <areber@redhat.com>