migration: mapped-ram: handle zero pages

Make mapped-ram compatible with loadvm snapshot restoring by explicitly
zeroing memory pages in this case.
Skip zeroing for -incoming and -loadvm migrations to preserve performance.

Signed-off-by: Marco Cavenati <Marco.Cavenati@eurecom.fr>
Link: https://lore.kernel.org/r/20251010115954.1995298-3-Marco.Cavenati@eurecom.fr
Signed-off-by: Peter Xu <peterx@redhat.com>
2 files changed