migration/multifd: Reduce access to p->pages

I'm about to replace the p->pages pointer with an opaque pointer, so
do a cleanup now to reduce direct accesses to p->page, which makes the
next diffs cleaner.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
5 files changed