c/r: rearrange things to pass struct migrate_opts all the way down

c/r: rearrange things to pass struct migrate_opts all the way down

If we don't do this, we'll end up changing the function signatures for the
internal __criu_* functions each time we add a new parameter, which will
get very annoying very quickly. Since we already have the user's arguments
struct, let's just pass that all the way down.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>