migration: Make threshold_size an uint64_t

It's always used to compare against another uint64_t.  Make it always clear
that it's never a negative.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240117075848.139045-2-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
1 file changed