docs(changelog): note the virtio-mem discard fixes

Record the snapshot-restore virtio-mem discard fixes: preserving
MAP_NORESERVE so the discard mmap cannot fail with ENOMEM, and mapping
each unplugged slot back to PROT_NONE so unplugged memory stays
inaccessible to the VMM.

Signed-off-by: Jay Chung <jaehoc@amazon.com>